I've had Cingular since they took over from Cellular One. On a scale of 1-10, I give them an 8. They have recently upgraded the styles of phones that they carry which is a definite plus. I have free nights and weekends, free long-distance, and no roaming anywhere!! For only $52 a month. The #1 problem that I have with them is that when I am outside of my calling area, missed calls and voice mails won't show up until hours later. When I go visit my family in NY, I won't get most of my messages until I'm on the NJ turnpike going back south. And their customer service up there is provided through another wireless serivce (ie. Sprint, or TMobile).
My fiance has T-Moblie and it totally sucks!! He has had to send back his phone for a new one and he still has issues using his call waiting, and other basic features like that. He looses a signal just by thinking about it, and the customer service isn't that great.