I am currently finishing up my first year in the program. If you search for my posts you should find a good deal of information regarding the program (good and bad). But to answer your questions...
In my opinion, the postbac student population is very supportive, but the school is definitely trying to weed people out. There are definitely significant numbers of people who do not finish the program. The classes you take are large lectures mostly (over 100 people for Chem and Physics), and you are fully mixed in with the undergraduate population. (I can only speak for first year though)