These prep courses are somewhat deceiving. From my impressions, their first diagnostic is incredibly difficult. Essentially, you're scared to death and study your ass off. Near the end of the course, after countless hours of studying and lectures, the diagnostic they give you is a bit easier and more like the actual MCAT. The effect is, because of all the study time the course motivated you to put in, your MCAT scores end up being relatively decent.
I was going to take Kaplan, but have decided I don't need to spend $1400 to have the motivation to study. You can buy the same materials they use for much, much less.