Actually, its AT&T that just got bought out by Cingular, not the other way around.
The AT&T network is kind of a mess because some old AT&T phones (like mine) weren't compatible with some Cingular GSM towers. AT&T gave a lot of people (like me) free 'upgrades' to work on the Cingular network, but besides that feature, the phones were really downgrades.
I don't know about Cingular up here, but down in Southern California their service was great. Cingular probably has one of the best coverage maps in the US, and now that they have access to AT&T towers, its even better.
T-Mobile, unless something changed recently, uses most of Cingulars network, so coverage with T-Mobile is similar to Cingular.
Verizon has got great coverage too, probably on par with Cingular, but I've only known a few people that use them.
Nextel seems to be the way to go up here. Their coverage can be a bit spotty, but the network seems fine here in Reno.
I have AT&T right now, but my contract ends in 6-7 months. Its okay. Not the best service, but decent. Cingular is a little bit better, IMO, but since they are now one company, we'll see how long it takes for AT&T to get a lot of the cool features that Cingular has.