Agreed with above, but I will add that I don't think prep courses are necessary to performing well on the test (and I'm a Kaplan teacher).
There are strong benefits that prep courses will offer you, but not all students will benefit equally.
If you would like a structured environment to help guide your test preparation, then choose a prep course. If you do not need guidance for prepping for the exam, then merely purchase the AAMC exams and use your textbooks for review.
An example of a student who would fall into the latter category: one who has achieved a high level of academic success (i.e. 3.7 GPA) in the classroom as well as in the lab (meaningful participation - not lab tech stuff) do not need a prep course.
An advisor should not be perscribing the same advice to all students regardless of differences in academic/lab background and ability.
Regards,
DJ