I decided against this personally, but there are a lot of people on here (and on mdapps) that have done this successfully. I noticed a pattern among those people:
1) they all said they felt extremely comfortable with orgo I. They were the kind of people that could do orgo I problems in their sleep. They understood concepts so well that they could extrapolate concepts to guess what would happen with reactions they had never seen.
2) they used books to fill in gaps in knowledge. EK, BR, whatever was needed and didn't really need the classroom to learn the concepts.
3) their MCAT scores in other sections were high too. This leads me to believe they are generally good test-takers.
As to why I decided against it personally: I'm not great at organic chemistry and I'm definitely not one to be able to fill gaps in knowledge by just reading up on them, when it comes to organic chemistry especially. I very much need the instructor there (part of the reason I also decided to sign up for a PR course). I have difficulty with reactions I have seen before, let alone tackling reactions I have not. I had a few more classes I wanted to take, and figured it was better to apply later with a strong score than a year earlier with a poor one. Also, my biology knowledge is not strong (I took intro to biology in 2002 and though I have a bio degree, almost all my bio courses were in neuroscience which isn't useful for the MCAT) and didn't think I could count on doing well on the biology questions.
Hope that helps.