I've had AT&T since they were AT&T the first time (before Cingular), and never had a problem. Lived in three states, travelled through many more, and I don't lose signal anywhere. My boyfriend and I were just discussing the end of his T-Mobile contract tonight, and he wants on my plan bad because AT&T has better phones, better customer service, and better service. (Their best phone is the HTC Fuze, for the record...especially if you already have an iPod.)
I've also had Verizon, and they were great...then they changed their customer service requirements in 2004 and I switched immediately. I hear it hasn't gotten any better since...