Much of the value in the Kaplan course is that it gives you a structured outline of what to study, and when. The amount of material on the MCAT is overwhelming at first (it's akin to studying for 10 final exams at once) so many people benefit from having a guide. Also, having somebody to answer your questions, in person, is a great resource you simply can't get from books alone.
(in the spirit of disclosure, I am a Kaplan instructor)
I took Kaplan myself, and I scored well enough (90th percentile) to teach for them, so there ya go. Take it as you will.