I think its just a personal peference. I spent the money and did an organized post-bac. It was worth it for me to just submerse myself in a program where everyone was working as hard as I was. As a result, I earned a very strong BCPM GPA at a very good school and made some great friends. Now, I am taking classes at a local state school while applying to medical school and I am not very motivated. Had I tried to do it on my own, I know my GPA would not have been as high, it would have taken me more than a year to apply, and my science classes would not have come from as highly ranked a school. Also, I have been told in interviews that my committee letter is excellent.
Having said all this, I am simply one of those people who is more motivated when I am in a structured program. I have two friends doing it on their own. One is going on her third year of trying to complete her pre-reqs and is having a very hard time getting into the schools she has applied to. Another is doing very well. You have to decide what is best for you and how well you self-motivate.