From the AAMC MCAT Student Manual:
Should I enroll in a commercial review course?

In preparing for the MCAT, some students enroll, at considerable expense, in courses offered by commercial firms in the belief that such courses will increase their knowledge of needed concepts and skills and improve their test-taking facility. The material in this Manual , along with your undergraduate academic experiences, should make enrollment in such courses for these reasons unnecessary.

Some students believe they will be at a disadvantage if they do not enroll because they think that those who do will have the opportunity to see and study questions that may appear on the MCAT. The promotional materials for some review courses imply that their courses do, in fact, offer such an opportunity. You should be aware that strenuous efforts are taken to ensure that no student is provided such an unfair advantage.

The results of a study comparing the MCAT performance of students who had enrolled in commercial review courses with the performance of those who had not, involving over 20,000 students during a five year period, indicate that gains derived from commercial review courses are small. The gains do not support the contention that review courses provide increased knowledge and facility in test taking. You should realize that the small differences in the scores of individuals receiving coaching may simply be due to the time devoted to reviewing relevant material.

While there is no one best way to prepare for the MCAT, a preparation regime that begins at least three months before the exam date may serve you well. Initially, as earlier suggested, you should begin with review of the reasoning, problem-solving, and writing skills described in this Manual . You should also review the science topics, using relevant introductory texts, class outlines and notes. You should pay particular attention to unfamiliar or new material. Early on, you may elect to work through the MCAT practice test to identify topics / skills needing additional review and to determine whether pace might be an issue for you.

Completion of the practice exam might be followed by continued review of problem areas, beginning with content areas not recently studied. If you have a study partner, you might each use your strengths to help the other address areas of weakness. If pacing is a notable problem you might determine whether your college or university offers study skills or reading skills support focusing on reading speed and comprehension.