http://en.wikipedia.org/wiki/HU-210 http://en.wikipedia.org/wiki/JWH-018 and http://en.wikipedia.org/wiki/CP_47,497 are the chemicals active in it.
essentially you're paying 25%-40% more for half the effects + a placebo effect.
I've heard mixed responses. Surely, it affects everyone differently, but everyone I've spoken to can barely differentiate the effects of Spice from weed. I'm not a regular smoker anymore, so it's almost the same thing in my book. I've been reading things about it creating a "dysphoria", but I experience that anyways with weed. Not that I completely bug out, but certain social or public situations can make me queasy while I'm stoned.