I have used Sprint and PrimeCo which is not AT&T I think. Originally had PrimeCo and switched to Sprint because PrimeCO SUCKED so bad. Then I got tired of Sprint's 9billion night and weekend and 13 minutes during the normal calling hours for 59$ a month or some BS like that. Sprint did have really good coverage though. Now I have AT&T and so far it has been good, the phones were cheap and have great talk/battery time. AT&T has a good plan for sharing minutes between phones for only 60$ a month.
Personally, I have Voicestream. I love it. I couldn't ask for more in a cell phone company. It's the only one company I've ever used, but I have a tiny experience with Cingular -- I was in the car with a friend when he dropped by a Cingular store to work out some bill problems . . . I spent 4 1/2 hours there waiting for them to sort it out, and it still wasn't right . . . My mother has Cingular though and it works okay for her. She never uses her cell phone anyways though.
Hey Vengeance, that was the reps fault. All companies have good and bad sales rep. They key is find a rep who you like and just get his mobile # and call him when ever you have a bill or account problem. I never call CS any more, I just call this rep who knows me by name and is helpful. BTW is cingular GSM in Houston yet?