The thing about the postbacc is that it's basically "do or die" in my eyes. Meaning that you need to step up to the plate and if you strike out then it's not going to bode well for you. OR there's even more that has to be done for damage control.
Otherwise, if you do well in the postbacc, no one can deny your success and it's that much more of an indicator that you can handle medical school.
Personally, I can attribute part of my success this application cycle to the postbacc so I see it as a positive thing.
I guess it could depend on the program. But many of the folks I know who took prereqs or upper level sciences at postbacs got into awfully good places. And the big name postbac programs all seem to have impressive med school placement track records, in part because they are a brand name commodity, and in part because their directors effectively market their students to deans at various med schools.
I don't think that's the case, but you are held to a higher standard. Generally post-baccs take less than a full-time courseload (8-12 units of all math and science classes), so you're expected to do well. A 3.7 undergrad GPA, for example, is stellar. A 3.7 post-bacc GPA is decent but not great, especially if you were only taking 2 classes per semester.