Well I switched to AT&T about a month ago and had the same concerns. I had them in the past(years ago) and did have trouble at times. Since I switched though I have had excellent customer service as well as reception.
I live in New Jersey but have traveled up and down the East coast since switching and have had no issues at all. I was surprised in fact that when in S.C. out in a remote area I picked up 4G service.
For all the complaints and bitching about AT&T having a crappy network and horrible service, I have not had any of it. The other thing I will mention is that my bill and plan are exactly what they have said it was going to be, unlike my experience with both Verizon and Sprint.
I have a friend that lives in the New York/Staten island area and he has had no issue either. Hope this helps. I can't speak for Chicago, but being a larger city I would have to say that I doubt there are issues there if they can cover the back woods of S.C.