I've had AT&T/Cingular since 2002/3. I've barely ever had an issue. When I did, it was one month where they did seem to run ******. Then that went away and I've not had an issue again *shrug* (Ok, once at a county fair where probably all the people conglamerated together in an area that usually isn't that populous probably overloaded the towers there. Actually, it turned out it was my iphone had crashed and needed to restart which has happened to me occasionally). I've used my phone in Washington, Georgia, Connecticut, Long Island, and New Jersey.
The only carrier I avoid like the plague is Sprint. And to be fair, maybe they've improved by now (to have still survived I would think so). And it wasn't dropped calls. It was so reliabley bad connection calls I could never understand anyone calling on Sprint. And everyone I knew with Sprint had the same complaints.
MY parents had Sprint and I finally asked them to call me on their landline cause I never could understand the call (and htis was the time Sprint was advertising that you would misunderstand people on other networks. My experience their parody of other networks fit them to a T).
My only thing with Verizon (once again they may have changed by now) is they were significantly more expensive than Cingular or T-Mobile (and Cingular had better coverage than T-Mobile which is why I went with them). Like by 20 dollars a month when I was shopping for plans (this was just regular voice plans). I've been happy enough with Cingular I've never really felt the need to change *shrug*. I probably would not have gotten the iphone if it wasn't on AT&T (cause I was just browsing phones AT&T had). And now I love the iphone so much AT&T would have to suddenly get really bad or another carrier would have to get really good (or a really enticing phone) to make me want to leave.