I don't know about the other prep companies, but I found Kaplan to be very helpful. The classes, in my opinion, could almost be considered optional. The materials were, on the other hand, invaluable. By the time I took the real thing I had taken 13 full length MCATs, not to mention hundreds of practice passages and practice items.
My advice is take the course, keeping up with the syllabus and online topical tests. At the same time, go to your Kaplan center 3 days a week. Take a full length exam every weekend and make sure you go over the explanations when you are done, so you know why you missed the ones you missed and got the ones you got.
Take Kaplan 1-5 when they are proctored, and save AAMC 4-6 for when the MCAT draws near.
Take it easy for a few days before the test. I went and saw the Kids in the Hall live. Talk about a stress reliever!
Hope that helps.