I looked into both options (doing post-bac at Harvard Extension or Boston University, and also a very structured program at Tufts. Tufts rejected me from their program, and I opted against BU and Harvard Ex... but got into med school anyway). In the end, I ended up completing the coursework on my own -- I had a bachelor's degree in German and a master's in Health Promotion, and definitely had other premedical science classes that I had to take!
Part of my decision was related to scheduling difficulties with the post-bac schools (as well as Tufts rejecting me from their post-bac program, obviously), and part of it was dealing with some of the people in charge of the programs. In the long run, I was able to register for the courses on my own, so I did. The only real drawback was that I could not use the schools' official application/letter of recommendation assistance, but I organized all of that application crap myself and was able to get plenty of letters from profs, bosses, and prior military commanders and medical officers.
If you need the structure, or think it would help you, then a post-bac program is a fine idea. Just look into the school and the program before you sign on. Some are really excellent, and some are poorly managed. If you are more of a self-starter, you probably don't need it. That is just my opinion, of course, but it worked out for me!
[ Edited by Corpsman-Up on 2006/5/16 22:43 ]