I looked at buying a co-op in NYC (very hard to find a condo) but there are huge landmines. As I mentioned in another post, maintainance fees can been huge -- some >1000/month which is not deductable. There is another hidden trap which is enormous with co-ops. Unlike a condo, you don't own your unit. You own a share in the building and the right to occupy your unit. If your neighbor defaults on his unit and it gets foreclosed, the rest of the residents have to make up the difference. This can cause dominoe effects in some buildings taking down all the residents. This is particularly dangerous is small buildings, because one empty place could raise your maintainance a lot. Another thing is that most co-ops have rental restrictions. So don't just figure on renting it out when you are done.
If you do feel you must buy a condo or a co-op get an attorney who specializes in these deals to represent you. This is not a simple real estate closing. The financial records of the building must be scrutinized.