Nope, you have a test to make the preparation and based on that you get a dicepool that you have to roll again to get your preparations result in action (which is why alchemy is somewhat less effective than using spells directly)
Depends on what you use the spells for. One of the biggest benefits of alchemy is that you 'pre-soak' the Drain, so you don't get bushwacked during combat. It also works wonders for unopposed, sustained spells, and AoE combat spells. Let me give you some examples:
Heal - Remember that normally you have to sustain the Heal spell until the effects become permanent, taking that -2 all the while. Since a preparation sustains itself, you can distribute talismans to your crew, and set them off if you need to, healing at range and not needing to worry about sustaining penalties while under fire. Less efficient than directly casting the spell, but the benefits in being able to heal the samurai from under cover, rather than going up to touch range (and passing under fire the whole time) and then having to sustain the spell multiple combat turns (not passes) to make it stick... well, that's fairly obvious, isn't it?
Increase Reflexes - This works especially well for street-level runners, or for the mage himself in higher level teams. An Increase Reflexes preparation sustains itself for a short duration, almost like taking combat stims, but without the whole addiction thing. And, again, you don't need to sustain it. Being able to give initiative boosters to a whole street-level team during combat, without stacking -2 penalties? It is a beautiful thing.
Armor/Astral Armor - Honestly, if I have to explain why having extra armor on command is a good thing, then you really don't need to be reading this post. The fact that you can get it without sustaining penalties is a good thing.
Chaotic World/Mass Agony/Opium Den - Really, any of the AoE illusion spells. These spells pump a flat -1 per hit on everyone in the area for the duration. Not needing to sustain them makes these things really good for ambushes, or essentially debuff grenades. Sure, you might only get a -2 to -4 penalty, but that's two to four dice that aren't going towards their attacks, or their spellcasting rolls, or their hacking attempts, or their... you get the idea. Moreover, 2 to 4 is potentially enough to take low to mid Willpower enemies out of the fight entirely during the duration. Especially if you combine with anything that saps Willpower, or (if your GM is generous) layering multiple effects of different spells together (Mass Agony and Opium Den going off in the same area, for instance).
Ice Sheet - Also, other spells like this one that provide AoE battlefield control. Even though the spell doesn't need to be sustained, it does not suffer from having a smaller dice pool, since you really only need 2-3 hits to have a major impact if you choose a good choke point to use it on. The benefit here is the lack of having to deal with drain in combat.
Fireball - Also, all AoE combat spells. Explosions on demand, that don't register on chem sniffers. Great for sabotage, distractions, ambushes, or terrorism (sneaking a talisman into a target vehicle is a lot easier than sneaking a bomb into one). And, depending on the type of trigger used, you can be safely very far away, preferably in front of other people who would serve as alibis, when your handiwork goes off.
Physical Barrier - A barrier thrown up by a preparation won't stand up to much punishment, but it will do so until it runs out, no matter what you might be doing. Besides the obvious part about blocking hallways and stairwells to discourage pursuit, it works very well at keeping people out of rooms, especially when the door swings inward. Place it inside, and anyone trying to get in has to blast through both the door (or surrounding wall) and THEN the barrier.
I could go on, but that's enough hints for now, yes?
Yes, you can do any and all of that with spells, but you'll be facing mounting -2 sustaining penalties, having to roll against Drain multiple times while in combat, and so on. It is a different playstyle from the typical SR mage, but if you know how to work it, it can be devastating. Just make sure to have a weapon skill instead of just relying on Flamethrower for all your killing needs.