I have VoiceStream/T-Mobile and it works pretty well for me. I haven't traveled very far with it, though, so I'm not too sure how much coverage they have.
The only time I really have any trouble with dropped calls is in my apartment
where I have very little signal... that's what I get for living in the ground floor of a cement block building though. However, I'm moving into a house in a few weeks, and there, I get great signal!