I've been a VoiceStream, then T-Mobile customer for over 16 years. There's been ups and downs in the relationship over the years. The most notable "down" was the $800 international roaming charge they refused to remove from my bill a few years back. Even though I really wanted to leave them then, a thorough cost analysis of the competition showed they really were still cheaper...

More recently, the Simple Choice plan they introduced last year which includes "free" international data roaming has ensured I stick with them for even longer. I travel quite a bit, so that + the wifi calling which works pretty much anywhere in the world has been a great thing.