Ok, I think I need to give you the flip side of Kaplan, after Camille's input. Not that I'm saying her's isn't valid - I just had a completely different experience with them.
I walked in to my first day kind of cocky about it; I'd done very well in all the pre-requisite courses and was actually trying to figure out what a prep course could provide other than a schedule by which to review. I ended up with QUITE the crappy score on the diagnostic (and yes, it's designed to give you a crappy score for various reasons) and decided that I was going to do my best to "buy into" the whole system. (For a skeptic, this was a pretty unorthodox attitude). So I did as much of the homework as I could, made it a habit of taking practice tests even outside of the proctored ones, and used the HECK out of all of the extra review material they provide (but do not necessarily require). I ended up going up 6 points in PS, 5 in Verbal, and 3 in Bio.
I guess for me it was worth it, but with the Kaplan program you really need to be able to self-motivate and actually DO the "homework" they ask of you. Otherwise, the in-class portions are nowhere near as effective.
I, obviously, didn't take Princeton Review, but one of my friends just took the MCAT for the second time - the first time he prepped with Berkeley, the second with Princeton, and in his opinion Princeton was FAR more effective than Berkeley. Berkeley, apparently, tends to focus on content more than anything else, which is pretty useless. From what i could tell from his prep materials, though, Princeton does cover some stuff with I never encountered on an AAMC practice test, OR the real one, and a lot of their homework and review questions aren't in the multiple choice format (which I, personally, think is sort of silly when prepping for a multiple choice test).
$0.02.