I did the postbac program at Columbia. I'm now in my second year of medical school so I can't really complain too much about the program. It did after all allow me to get into medical school. A few words about the program. They take just about everybody who applies with an undergrad GPA over 3.0. The program is a moneymaker for the School of General studies and be certain that as a part time student you will be low on the university's list of priorities in so far as advisement, class choices, etc. The program is difficult, but at the end of the day, they are still just undergrad lower division science classes you will be taking, and at half the load of the Columbia College and Barnard students. If you do choose CU, I would advise you to take morning, not night, classes, as you will be graded on a curve with undergrad students who have more classes to study for than the typical postbacc student taking one or two classes at night and who has all the time in the world to study.
Personally, if I could do it over again I would have gone to another program that was less expensive. Columbia does get a high number of applicants into medical school. However, I don't feel the program is what it promotes. I did reasonably well in the program (GPA 3.3 and 32 MCAT) and I only received acceptances to two lower tiered medical schools. I'm happy where I am so I can't complain too much, but I can't help but think that with my MCAT that if I had gotten a 4.0 at a community college or a less expensive state program I would have landed in the same place I am today, but with $25K more in pocket.
Lastly, the attrition rate at CU is high (>40%), but keep in mind that the majority of those people choose to leave instead of flunking out. Most didn't realize the full scope of the long road ahead of them to becoming a doctor when they started the program. I would say that most people who enter could successfully complete the program. It's all about effort, less so that intelligence.