OK, my two cents on possession in particular:
There seems to be this idea that a Possession Trad Mage can call down a Force whatever spirit into himself and gain mad bonuses. I agree. However, there's this other idea that the Mage is running around as himself in full control of his body with these bonuses. I beg to differ. Once you're posessed, your body is not yours. You retain awareness, yes. The spirit has a certain number of services to fulfill, yes. That's not the same as driving. You're not jumped into the drone anymore, you're watching through the drone's eyes while your sprite controls it. It just so happens that the drone in this case is your very own, one of a kind, not replaceable body. Chummer, you just handed your body over to a living archetype. Don't think he's going to do things your way.
Spirits in a possession tradition have their own powers, but they also have strong motivations and personalities. What they are not known for is suppressing their own personalities to spare the person they're riding. Look up some voodoo loa and ask yourself if you really trust Erzulie to negotiate a contract for you. If you want some specific price, or some bit of gear, she has to obey and go for it. That's a service. If you want her to NOT walk your body into the back room with Mr. Johnson for a night of wild sex, that's a service. Spirits don't worry too much about consequences, either. Chango is not going to hide from Knight Errant when he can face them honorably. Baron Samedi doesn't think death is a big deal. He's going to kill someone or leave them alive based on what's funny to him. And every last one of them is going to leave the posession mage with the bill for cleanup.
On the other hand, you can hand over someone else's body. Its not as fun, not having all those bonus dice but you do have a friend with them and clean-up is his problem.
Edit: Unless you have Channeling Metamagic. Then you've got two people jumped into the drone, you and the spirit. If you don't want that to turn into a contest, you'll still give the personality of whatever is possessing you its due and all mental resistance is based on whoever has the worse stat.