I had a few friends take the Kaplan course. While they did improve their scores by like 5-6 points, they started out pretty low (we're talking hovering around 20). And these people are smart, 3.5+ GPAs, doing some quality bio research, just not good standardized test takers. It's a damn shame.
The people I've known to get the highest scores just used books. I would also state that a lot of these people are naturally gifted; they study less and can get good grades. One got a 36 or so buying the Kaplan book off the shelf. I tried to read the damn thing and I felt like I should have just borrowed text books because they at least have pretty pictures and those little info boxes with 'FYI' crap that's only useful in Jeopardy.
On the other hand, I did use books from Examcrackers (it can be found at Amazon or elsewhere on the Net) and I really liked their material. Picked the idea off from this forum. They cut the... crap, so to speak, and focus on the test stuff, which is extensive enough. They also write in plain language, which makes deciphering concepts much easier. With that and a little prayer, I got out OK (hehehehe.... supernatural forces had to be working because I kept hovering between sleep and consciousnesses in the bio section- long story).
And Examcracker has Salty the Cracker for a mascot. Not quite Mr. T, but it'll do.
In essence, what you do depends on who you are. All the people I've mentioned above have very little time due to work, athletics, etc. The 36 guy had brains and made time (either an hour a weekday for 3 months... his roommate said he just geeked out on Final Fantasy... the truth is out there... somewhere in between). This guy also made an A in zoology while picking on his fingernails as I scrambled to take notes in class.
If you have sufficient time, and you do good on standardized tests- this applies especially to the verbal section-, just get some books you'll like and actually read, do some study (there is route memory on the MCAT), definitely do practice tests (but the majority of the MCAT is synergizing concepts with the reading passage they give you), and kick the, uh, crap out of the thing.
If standardized tests (my opinion they're crocks) just nail ya, or you don't have time to make yourself self study, then I would make the sacrifice and go for the Kaplan thing. Kaplan will require you, however, to devote time outside of class to study, depending on what your schedule is.
Combining time with effort and correct, focused study from whatever source, you'll beat the hell out of the thing.
But then you meet that girl (or boy), or you got that orgo exam with a physics one next week, or Fox is showing 'Who Wants to Marry a Shallow, Materialistic Publicity Hound?' ...
P.S. If ya ain't got any of those, try the prayer thing. No one's really quite pegged which is the best, and that includes voodoo or the Hale-Bopp Comet suicide cult led by D'oh. Trust me, after taking the MCAT, you'll want to perform the former on med school admissions, and your brain will feel like it's been under the rule of the later. If you're not into prayer, cults, work, or have super brains, then I would start looking into contacting organized crime or dumping $$$ into your dream school. I personally bought Tulane the Dancin' Homer Human Excretory Facility, built under the auspices of Port-a-Let. I think Harvard will want more.