Just anecdotal experiences from friends,
but if you're talking about the big 2 (Kaplan and Princeton Review), I've heard that Kaplan is better if your science is a bit rusty, and Princeton Review has better pure-test taking strategies (although I've heard Kaplan's aren't that bad either). Princeton Review definitely has better verbal prep (their verbal workbook is supposedly amazing), so if you take Kaplan, I'd look in the for sale forum on here or on eBay and pick up a copy of the Princeton Review verbal workbook to supplement the Kaplan course.
Obviously people have different opinions on it. Some swear by Kaplan and some swear by Princeton Review. Other people self-study with only Exam Krackers or only Berkeley Review, and some self-study with both. Still others will throw in Nova physics just for help with that subject and then use Princeton Review for verbal along with Berkeley Review for the four science subjects. I think the most important thing in the end is not what course you pick or what books you choose (because all of them are at least decent), but the time and effort you put in. Everyone has a peak score that they are able to reach, obviously (most people could never score a 40 even if they spent 2000 hours over one year prepping for it), but AAMC recommends at least 250 hours studying if I remember correctly. I don't think that's the amount of studying that will peak most people's score, however. If your knowledge of basic science is near flawless then perhaps 250 hours will get you to your peak, but for a lot of people (myself included) who have flaws in our science content knowledge, I think 450-500 hours is probably the amount of studying that will allow us to reach our peak scores. The studying has to be targeted and efficient as well -- you should do practice tests regularly (AAMC practice tests are best for this, especially the later ones (7-11) but I've heard good things about Berkeley Review and Kaplan as well), and you should keep track of what types of questions you're missing in what subjects, so you can focus on your weak areas to boost your overall score. If you're taking the test July 6th, that gives you 63 days starting tomorrow. That's 9 weeks. If you take one day off per week, that gives you 54 days. Figure on at least 8 hours per day and you'll be somewhat near 500 hours and probably near your peak. I'd spend the first 6 weeks doing exclusively content review with lots of practice questions (with some Kaplan or BR practice tests mixed in every 4-5 days) and the last 3 weeks doing two AAMC practice tests while shoring up your weak areas that the AAMC tests expose. That's my plan, and I'm planning a July test date as well.
