The Faerie seem to be much more blatant about compels and their influence in the books. Even the influence of Hellfire was more subtle (grumpiness/irritability/unfriendliness). Soulfire is more subtle still. In the books it is almost done like some sort of feeding dependency (where rest and relaxation are what you need to get to recover). In the books if someone uses Soulfire, he/she is literally tossing bits of his/her soul at enemies. If they do it a lot, then they seem to get really drained emotionally and physically. In a lot of ways, it isn't written like sponsored magic of other types at all.
If I were running with Soulfire, I'd say what it can compel if you wrack up debt are rest, time with friends and family, and helping people out....if we were to go like the books, one would want to be very subtle about its influence. It isn't written as the type of thing that blatantly forces you to do something. The rest and time with friends helps your soul recover so that isn't forcing anything. As for helping people out, it seems to help make [someone in the books] a better person (like Hellfire seems to make people more grumpy/irritable/etc).
I think there's a real trick to making this properly subtle. Soulfire's influence is almost completely internal if you go by the books ([someone in the books gets the power somehow] but how it changes their behavior doesn't make it look like anything external is pulling their strings). Part of doing this right, perhaps, is having the player committed to making the influence of Soulfire actually part of the character's internal motivations; using it just makes you a better person (or emphasizes your already good qualities).
I suppose one could do this with any sort of sponsored magic, but it seems particularly true of Soulfire. I'm making a Son of Thor character for an upcoming campaign (I posted on this here a good while ago, but we ended up doing a Star Wars campaign first). I've toyed with the idea of him having some sort of "sponsored" magic that is wholly part of him (the debt and such would still be there, but it would all be colored as internal motivations and compels), though I think I'll just start him out as a focused practitioner for now (need some points for inhuman strength). Like someone said, sponsored magic is very flavorful and adds a lot to the game.
