I took Kaplan and was happy with the structure of the course, the materials they offered, and the results. Would I recommend it to others?: definitely.
I would imagine that with whatever MCAT prep-course you choose to take that the effort you put in will ultimately dictate the results that come out. That being said, I absolutely could have put more effort into my MCAT preparation or re-arranged some priorities and probably have scored a bit higher.
Again, I think a lot of it revolves around your effort and committement. The fact you are going to take a prep-course (any of them) puts you farther ahead of those people who choose to wing it and study solo.