The prep class is going to be a lot more useful as a review rather than the first time you study the material. I really believe that having taken all the prereq's before the MCAT is the best way to go. That way, you follow Diana's Formula for Success: study 'em in class, then you review them in your prep class, then you rock the MCAT. The preps go so fast you barely have time to give them a solid review, much less learn the material from scratch, and that's even the prep class that's four months long.
As far as reviewing on your own before class, nah. That's why you're taking the prep.