Pretty cool.
Yeah, I'm a legacy AT&T Wireless, and now Cingular customer and I can tell you that the Cingular GSM roaming around here (which has been enabled for a while) REALLY helps. One disadvantage of it was that AT&T was billing mobile to mobile calls as anytime minutes if you were roaming on Cingular, but now mobile to mobile is free anywhere, structured like Cingular's plans. The roaming really helps out in buildings since Cingular owns the 850MHz GSM license around here and AT&T only operates on 1900.
I really love my service though, and my plan can't be beat. No one uses AT&T around here because their reputation is so bad, but there are a good handful on Cingular. All the rich ****s use Verizon because they think it's so good. I personally think it's horrible. My girlfriend has it and I routinely get service where she doesn't on AT&T/Cingular.