AT&T's service hasn't been as good as Verizon's, but it's not as terrible as people say. In bigger cities with more iPhones per capita their network speed/quality can suffer, but in Charlotte it does just fine. I have found only a couple of spots where my old Verizon phone had service but AT&T does not, but these are mostly inside buildings, and are reasonably small areas. On the whole, I don't think that I have suffered much from the switch.
tl;dr: AT&T isn't the best, but my experience cannot explain the enormous amount of whining that I hear from others. It's fine.