I am currently about to take finals that mark the end of my first year at the program at Columbia. It was not my first choice, but its where I ended up. Here's some basics:
cost: its up there... postbaccs pay by the credit, which is $1100 or so. If you havent taken any of the prereqs you can be looking at taking somewhere in the range of 10 credits a semester, so you can do the math...
linkages: don't know much about it as im not planning on doing it, but I know its really hard to qualify, and a lot of students who have the grades to qualify don't do it because they think they can get into better (or other) schools
attrition rate: the number that we've all heard is that 40% of people who start the program don't finish. Now, not all of those people are dropping out because of the support issue. Some go elsewhere because of the cost. Some leave because they arent making the grades. Some decide this just isnt what they thought they were getting themselves into. Who knows. But that's the number I've got.
grades: this is what we're there for after all, like it or not... in my experience, the classes have been very hard, and I've been working my ass off like I never knew I was capable... that said, the curves are fairly high... (at least in the classes I've taken so far... someone correct me if orgo/bio is different please). Average for a class tends to end up B/B+. Not that that makes it easy, but generally the curve can be your friend.
My general feeling would be to go with your gut. Columbia's got the name, sure, but it isn't everything, and in the end if you perform well in a postbacc, you'll more than likely go to medical school.