Always had great service with AT&T with the 4S and before that. Speeds have been good too, except when watching a baseball game, but I figure that's because you have 15,000 or more other people right around you, etc.
Excited to try LTE in the city... next time I visit... and when I get a 5, they're really still hard to get a hold of without ordering from Apple.com! (I want no contract pricing and you can't do that from Apple.com right now)