I had sprint which I thought worked really well, but I was jump-roping in the ocean one day with a piece of seaweed and I noticed my phone floating away in the ocean - my social life soon died there after. Since I'm going to Japan in a month, my mom let me borrow her phone in the mean time, which is AT&T and it BLOWS. I hate their service, so I dont suggest them. Nextel would be cool, but only if everyone you know has nextel. If you dont know anyone, you cant remote them or anything, so what's the point? Also, I've heard horror stories on how you dont have any privacy cuz anyone can remote you whenever. So, it could potentially be a double sided sword. Althought I have never used them, I would highly suggest T-mobile. They're HUGE in europe - everyone has them. And I believe if you bought a phone here, you could use it there - no problem. Like I said, they're huge in europe and I believe they will be huge here too.... one day.
If I were not going to Japan, I'd get T-Mobile.