well, i debated long and hard about whether or not to take a course, but the $$$ was just too much, so i decided to go it on my own and see how i did before i invested. i took the test last summer and got a 38R. here's what i did: i started studying about two months in advance; i had it relatively easy because i took the august MCAT and was only taking one summer school class w/o working (even so, i admit i slacked a lot, so it wasn't like i was studying every spare second i had, hehe).
i went to the public library and got every MCAT practice book they had and did as many practice tests as i could. i also bought several of the kaplan books (including MCAT 45, their book of super hard questions, none of which i could actually answer, and virtually none of which type did end up on the exam). i registered for the AAMC practice MCAT online. i think i took the free princeton review online test. and i did review my class notes from general physics and high school AP chemistry. basically, i got a feel for what type of material is asked based on the practice tests and then reviewed only that stuff. i thought kaplan's huge MCAT course book had some good starting points for subject review.
i do have to say that what threw me, studying on my own, was that some practice books gave me the wrong impression of what the real exam was going to be like. for instance, based on the practice problems i did, i thought i needed to memorize a lot of physics formulas, when the actual exam i took was more concept-based--stuff like, "extrapolate from this experimental setup" rather than calculating flux or whatever.
of course, i can't say whether i would have done better than i did if i HAD taken a course, but i think if you can get a hold of some good quality problems and have a good grasp of basic science concepts, you should be alright without.