[MercilessMing]

Didn't really read the section on Invocation and Great Form spirits with a critical eye until now.  I want to try to make sure I understand it right.  The rules are divided between the Spirits section and the Metamagic section which makes it more difficult to comprehend.  Also I want to poke holes in it because I'm seeing some exploitable things.

As I understand it, to summon a Great Form spirit you (obviously) need the Invocation metamagic.  Here is the rules block in Invocation on pg 122:

Summoning a great form spirit is very similar to summoning a spirit normally, except as part of the same Major Action, the magician must invoke the spirit by making a Conjuring + Magic + Initiate Grade vs. (Force x 2) Opposed test; note that one of the dice added to the pool by the magician’s Initiate Grade is always a wild die. If the magician gets any net hits, those are added to the net hits from the summoning test to determine how many services the great form owes. If the magician does not get any net hits, the great form spirit goes berserk and attacks anything and everything within (Force x 10) meters for a number of combat rounds equal to the spirit’s net hits. Either way, the
magician must resist drain equal to the number of hits (not net hits) the spirit rolled. This is Stun damage unless it is higher than the character’s Magic rating, in which case it is Physical. If the drain is enough to knock the summoner unconscious (or dead), the spirit goes berserk for additional 1D6 combat rounds.

I take this to mean that summoning a great form is two tests wrapped up in one major action:
The Summoning Test
The Invocation Test

Only the Invocation test is detailed here, and it mentions a Drain Test.  So it's actually:
The Summoning Test
The Summoning Drain Test
The Invocation Test
The Invocation Drain Test
(Two opposed tests and two simple tests just to do one action is not good design IMO)

It's unstated, but it seems logical to me that if the Summoning test fails, we don't go on to an Invocation Test.  If the Summoning succeeds and the Invocation fails, the spirit always goes uncontrolled and berserk.  If both succeed, the summoner's net hits from each test are added together to get the total services that the spirit owes.

To figure out what is great about a Great spirit, we need to go back to pg. 66.  There appear to be zero mechanical differences between regular and Great spirits, except for access to a special selection of Powers.  Unlike regular summoning, Great Form powers appear to depend on the number of hits gained by the summoner!  That's according to the table on pg 66 titled "Great Form Spirit Hit Results".  However, this table is not referenced by any of the text.

# OF HITS RESULT
4 Gain Magical Guard power (p. 69)
5 Gain an additional optional power (see powers listed by spirit type, pp. 147-8, SR6)
6 Gain greater spirit power appropriate to the spirit type
7 Astral Gateway power (p. 66)
8 Endowment power (p. 67)
9+ One additional power or increase Force by one with each hit beyond this amount

It's very beneficial then that Summoning and Invocation add to give us the total services the spirit owes.  Since this is the final net hits #, I assume this is the number to use with the Great Form Spirit Hit Results table.  No text references this table so I have to make that assumption.  I also assume this table is cumulative.  If I get 4 net hits the spirit has Magical Guard.  If I get 8 net hits, the spirit has Endowment power and the powers from the lower results.  I assume this because if the spirit ONLY gets the result of the net hits I got, this whole endeavor would feel extremely random.  Also, if I got less than 4 net hits, my spirit is exactly the same as a regular spirit!  Bad design.

Great Spirit powers have failures in other ways:
Divining power says it's available to Great Spirits but the only time it could be applied is on the 9+ hit result or GM fiat.
If you get 6 hits on a Kindred Spirit, you get Magical Guard, which you already would have received on 4 hits.
I'd recommend fixing both of these by making Divining the 6 hit result for Kin.

Exploits:
Because Powers are based on the number of net hits you get, the obvious exploit is to summon very small Force Great spirits.  The fluff about Great spirits describes them as being bigger more powerful versions of regular spirits, but you summon them the same way.  Pick a Force, summon against FX2, then an invocation test against FX2.  Summoning against a F1 spirit gives you the best chance of getting access to powerful unique spirit powers like Astral Gateway.  Astral Gateway doesn't care what Force the spirit is.  Neither does Endowment.  Exploiting this way is actually a really smart way to get a high Force great spirit.  If you get 13 net hits against your Force 1 spirit (not unheard of, if net hits across both tests are combined, which I think they are), then you actually summoned a Force 6 great spirit for the price of a Force 1.
Endowment seems like another potentially exploitable power.  I'm not sure how to interpret this text:

This power allows a spirit to give access to one of their powers to a target. The spirit retains the ability to use the power. A character is limited to only a single power at any given time.

There is no mention of limits for how the target may use the power endowed by the spirit.  Two cases make the most sense though - either the target can use the power as often as they want, as limited by the description of the power, OR each use of the power by the target consumes one service of the great spirit.  Since nothing is said, I assume they can use it as often as the power allows just like a regular critter. 
It's also not clear to me whether powers granted in this way would use the spirit's stats for tests, or the target's stats.  I can see arguments for both and I don't know where to land. 
There's also some open questions I think.  Could an astrally projecting character Materialize if they were granted that power by a spirit?  Could it grant Sapience to a nonsapient target?  Can a spirit endow a target with Endowment? 

Reading this I feel like I opened up a can o' worms.  Discuss!

[Stainless Steel Devil Rat]

Didn't really read the section on Invocation and Great Form spirits with a critical eye until now.  I want to try to make sure I understand it right.  The rules are divided between the Spirits section and the Metamagic section which makes it more difficult to comprehend.  Also I want to poke holes in it because I'm seeing some exploitable things.

As I understand it, to summon a Great Form spirit you (obviously) need the Invocation metamagic.  Here is the rules block in Invocation on pg 122:

Summoning a great form spirit is very similar to summoning a spirit normally, except as part of the same Major Action, the magician must invoke the spirit by making a Conjuring + Magic + Initiate Grade vs. (Force x 2) Opposed test; note that one of the dice added to the pool by the magician’s Initiate Grade is always a wild die. If the magician gets any net hits, those are added to the net hits from the summoning test to determine how many services the great form owes. If the magician does not get any net hits, the great form spirit goes berserk and attacks anything and everything within (Force x 10) meters for a number of combat rounds equal to the spirit’s net hits. Either way, the
magician must resist drain equal to the number of hits (not net hits) the spirit rolled. This is Stun damage unless it is higher than the character’s Magic rating, in which case it is Physical. If the drain is enough to knock the summoner unconscious (or dead), the spirit goes berserk for additional 1D6 combat rounds.

I take this to mean that summoning a great form is two tests wrapped up in one major action:
The Summoning Test
The Invocation Test

Only the Invocation test is detailed here, and it mentions a Drain Test.  So it's actually:
The Summoning Test
The Summoning Drain Test
The Invocation Test
The Invocation Drain Test
(Two opposed tests and two simple tests just to do one action is not good design IMO)

I'm in agreement with you thus far.

It's unstated, but it seems logical to me that if the Summoning test fails, we don't go on to an Invocation Test.  If the Summoning succeeds and the Invocation fails, the spirit always goes uncontrolled and berserk.  If both succeed, the summoner's net hits from each test are added together to get the total services that the spirit owes.

Here's where the pedantry comes into play. "WeLl AcKsHuALLy", the language says that you add the services from the invocation to the services from the summoning, therefore if you get 0 or a negative number of services, you can theoretically still end up with at least 1 service after a good invocation result.  Although... I'd probably play it that if you get <1 service on the summoning, you go directly to Fail Jail without passing go or collecting your dice to roll Invocation.

To figure out what is great about a Great spirit, we need to go back to pg. 66.  There appear to be zero mechanical differences between regular and Great spirits, except for access to a special selection of Powers.  Unlike regular summoning, Great Form powers appear to depend on the number of hits gained by the summoner!  That's according to the table on pg 66 titled "Great Form Spirit Hit Results".  However, this table is not referenced by any of the text.

# OF HITS RESULT
4 Gain Magical Guard power (p. 69)
5 Gain an additional optional power (see powers listed by spirit type, pp. 147-8, SR6)
6 Gain greater spirit power appropriate to the spirit type
7 Astral Gateway power (p. 66)
8 Endowment power (p. 67)
9+ One additional power or increase Force by one with each hit beyond this amount

It's very beneficial then that Summoning and Invocation add to give us the total services the spirit owes.  Since this is the final net hits #, I assume this is the number to use with the Great Form Spirit Hit Results table.  No text references this table so I have to make that assumption.  I also assume this table is cumulative.  If I get 4 net hits the spirit has Magical Guard.  If I get 8 net hits, the spirit has Endowment power and the powers from the lower results.  I assume this because if the spirit ONLY gets the result of the net hits I got, this whole endeavor would feel extremely random.  Also, if I got less than 4 net hits, my spirit is exactly the same as a regular spirit!  Bad design.

Another pedantic point of divergence here.  There's a world of difference between hits and net hits. It's certainly possible that the intent is "net hits", but from a strictly RAW point of view it says hits.  And more than that, I'm thinking "hits, not net hits" could plausibly be the intention.  For the 4 hits threshold you noticed, as well as the chart assuming you might ever reach the 9+ level of hits.

Great Spirit powers have failures in other ways:
Divining power says it's available to Great Spirits but the only time it could be applied is on the 9+ hit result or GM fiat.
If you get 6 hits on a Kindred Spirit, you get Magical Guard, which you already would have received on 4 hits.
I'd recommend fixing both of these by making Divining the 6 hit result for Kin.

Bear in mind that the player doesn't pick what great form spirit power(s) are gained. And the GM is given explicit permission to fudge what power(s) are possessed rather than sticking strictly to the chart.  Ergo, if you summon a greatform spirit that already has Magical Guard anyway, the GM is not just allowed but I daresay encouraged to give some other power in lieu of that.

Exploits:
Because Powers are based on the number of net hits you get, the obvious exploit is to summon very small Force Great spirits.

Hits.  Not net hits.  The number of hits you can score is dependent on your dice pool, completely independent of how many dice the spirit is throwing.  Of course, that correction doesn't stop you from summoning a low force "great" spirit anyway, so that you can score a truly obnoxious number of services after you combine the results of both tests...

The fluff about Great spirits describes them as being bigger more powerful versions of regular spirits, but you summon them the same way.  Pick a Force, summon against FX2, then an invocation test against FX2.  Summoning against a F1 spirit gives you the best chance of getting access to powerful unique spirit powers like Astral Gateway.  Astral Gateway doesn't care what Force the spirit is.  Neither does Endowment.  Exploiting this way is actually a really smart way to get a high Force great spirit.  If you get 13 net hits against your Force 1 spirit (not unheard of, if net hits across both tests are combined, which I think they are), then you actually summoned a Force 6 great spirit for the price of a Force 1.
Endowment seems like another potentially exploitable power.  I'm not sure how to interpret this text:

This power allows a spirit to give access to one of their powers to a target. The spirit retains the ability to use the power. A character is limited to only a single power at any given time.

There is no mention of limits for how the target may use the power endowed by the spirit.  Two cases make the most sense though - either the target can use the power as often as they want, as limited by the description of the power, OR each use of the power by the target consumes one service of the great spirit.  Since nothing is said, I assume they can use it as often as the power allows just like a regular critter. 
It's also not clear to me whether powers granted in this way would use the spirit's stats for tests, or the target's stats.  I can see arguments for both and I don't know where to land. 
There's also some open questions I think.  Could an astrally projecting character Materialize if they were granted that power by a spirit?  Could it grant Sapience to a nonsapient target?  Can a spirit endow a target with Endowment? 

Reading this I feel like I opened up a can o' worms.  Discuss!

Since the rules ON the "great form spirit hit results chart" establish that the chart is, to paraphrase Pirate Captain Barbarossa "more of a guideline than an actual rule".... GM already has full power to nip abuse in the bud.  If you don't want to houserule that "great" form spirits must be at least X force, then just don't give them any great form spirit powers (since you have full control over what they have and don't have) no matter how many hits are scored.

[funkytim]

Check out what Sphinx said in this thread.  He basically said that hit chart really shouldn't be there for 6E.  After rereading the powers and charts I'm not sure that other chart should be there either because some of those powers are not listed as great form powers.  Here is the thread.

https://forums.shadowruntabletop.com/index.php?topic=32806.0

[MercilessMing]

So it's not clear what purpose, if any, the Hits for Powers table has.  Sphinx said this in the other thread you referenced

In addition to the standard and optional powers for their spirit type, great spirits start with an additional great spirit power according to the "Great Spirit Powers" table in Street Wyrd (p.66). Depending on what type of spirit you summon, this great form power could be Storm (Air, Fire, Guardian, Water), Paralyzing Howl (Beast), Quake (Earth), Temporal Displacement (Guidance), Magical Guard (Kin), Regeneration (Plant), or Wealth (Task).

I think you're mistaken, Sphinx.  Nothing that I see says that Great spirits start with that additional power.  The Great Spirit Powers table is not referenced anywhere in the text.  The only reference I can make out is in the "Great Form Spirit Hit Results" table on #6.  If it was intended the way you said, it's missing.

I generally like Street Wyrd but Invocation/Great form got bungled.  I can't say this is the most needlessly complex system possible to achieve the goal, but let's be honest - it's bad.  Invocation calls on an incredible four tests smooshed into one major action.  Why not introduce an inbetween step at least?  One major to summon, one major to invoke. 
What bothers me most is that it's not only complex but the mechanics don't really support the fiction.
The use of the Wild Die is interesting in Invocation but the Wild Die doesn't indicate difficulty, it indicates randomness.  You get the possibility of failure but also the possibility of amazing success (weighted slightly toward amazing success).  Thinking on this more, I would welcome a rule revision/house rule where a hit on the wild die adds a power to the spirit instead of adding services.  I wish the wild die provided for flexibility in the reward (GM fiat notwithstanding). 
Anyway, two drain tests sounds like an appropriate way to raise the stakes when we're dealing with Mega Spirits.  I would prefer ONE extra hard drain test over two regular ordinary drain tests though.  Fewer rolls, higher difficulty. 
Adding the net hits from each test together is counter-intuitive.  Why should the act of summoning a Great spirit typically result in twice as many services being granted than your average summon?  This mechanic doesn't support the fiction.  I'd house rule this too - net hits on the Summoning would grant services, net hits on the Invocation would only give services that could be used to stave off Banishing:
Example: Summoning - 3 net hits, Invocation 4 net hits.  Spirit appears with 3 services.  Banishing attempts would treat the spirit as having 7 services, and actual services owed wouldn't be reduced until after you eliminate those first four.
This fits the fiction better IMO. 

What may be worse is that the Great spirits themselves aren't that Great.
Regular spirits and Great spirits have identical stat lines and powers until you get to the special powers available to Great spirits.  So immediately it's disappointing that there's nothing inherently beefy about them.  In straight up combat, they're no different than any other spirit. 
Thankfully, the powers ARE great.  Storm, Quake... yeah.  Those are going to make a big mess.  So the whole reason to summon a Great spirit comes down to the utility of the Great powers they come with.  This changes their flavor from bigger, meaner versions of regular spirits to specialized utility spirits.  There's no real reason to summon a Great spirit until you need a power that only a Great spirit has. 
But they weren't even clear on how to assign powers.  Either you summon small spirits so you get lots of hits which means lots of powers, or they just get their one special Great Spirit Power, or the GM makes it up, or some combination of those.

[funkytim]

One reason to summon one would be great form channeling.  Talk about adding more rolls though.