I used to use cingular and it was OK... one of the only providers to have decent service at my house actually... I have heard that since the merger with AT&T, there have been alot of additional problems.
I've personally got Nextel, but honestly the service is hit and miss at best, and I really don't use the walkie-talkie services enough to really justify having it. BUT, their phones are the only ones that I can hear the ringer on, and able to hear the person on the phone while driving my car... so i'm stuck with them for another 1.5 years or so.
