I am a current 2nd year Columbia Postbac student (soon to graduate), and unfortunately I have to disagree with Pietachok.
The committee letter merely reaffirms how you did in the program. They rank you on a scale from highly competitive to not too competitive. One of the most important factors they take into account for this ranking is overall GPA in the program - the higher the GPA the better the ranking. In the end, as I said, the letter only reaffirms what your transcript says.
The advising is there if you ask for it. The advisors really not force you to come in and talk, you have to schedule your own meetings. If you do take the time to talk to/get to know your advisor, you will find out that they do have generally good advice. Keep in mind that preferences for things such as extracurriculars and types of volunteer experience can change from med. school to med. school, so good advice for applying to one med. school may not be the best advice for the next. The advisors are there because they know what they're doing and have helped hundreds of students get into med. school; at the same time, I have also done my own research to find out what the med. schools I want to attend prefer.
Biology at Columbia is a weeder class. This professor has been around for a long time and most of the deans of admissions know her personally. The average of this class is between a 70-74, which is CURVED to a B+. It is a very difficult class - Prof. Mowshowitz teaches you the material, and then on an exam she gives you a passage about some sort of disease and you have to figure out what's going on (sound familiar? MCAT?).
Out of 6 professors, I have never had a professor who did not want to write me a letter of recommendation. Same goes with all of my friends who have had different professors. Enough said.
See the FAQ: many non-traditional students DO succeed:
http://www.gs.columbia.edu/postbac/admissions-faqs
My background was Psychology at a non-Ivy League school. I AM slightly incompetent at the sciences, but I am doing very well in the program due to the effort I am putting in. As I have said on all of my other posts, what you put in is what you get out.