Since you asked, I live in Chicago and coverage is wonderful here: very few dropped calls and great 3g service. When I leave town, admittedly, it gets more spotty. Coverage is a YMMV thing with any carrier.
T-Mobile also has great customer support and serves my unlocked Nexus S without a contract and with free tethering, something I'm sure AT&T will refuse to do.
PS: I used to have AT&T (actually cingular back then). Service in Chicago was atrocious. Any phone call over a few minutes would drop at least once, I'd have to stand in the window of my apartment to make phone calls, etc. Once again, YMMV.
T-Mobile also has great customer support and serves my unlocked Nexus S without a contract and with free tethering, something I'm sure AT&T will refuse to do.
PS: I used to have AT&T (actually cingular back then). Service in Chicago was atrocious. Any phone call over a few minutes would drop at least once, I'd have to stand in the window of my apartment to make phone calls, etc. Once again, YMMV.