Virtually ALL of my MCAT review was from online sources, mostly wikipremed, khanacademy, and mcat-review.org. I also made my own O-chem flashcards from a Gold Standard website. I looked over my old O-chem notes once the last week before the test. The only additional science course I have taken besides the pre-reqs is A&P, and my bachelor's degree is in theatre.
Result? 14/11/14. IMO, you do NOT need to shell out money for "big name" test prep material, especially just for content review. It's out there. Practice questions and practice tests, on the other hand, I do think you need to suck it up and purchase.