Which kinda makes T-Mobile the only real option if you have an unlocked phone. Why spend all that money on the phone if you're still going to be paying to subsidize something you didn't buy? As long as you're happy with the service, of course.
(Currently stuck on AT&T.)
http://www.pcmag.com/article2/0,2817,2357692,00.asp
"T-Mobile, AT&T Push HSPA 7.2-Mbit/s Upgrades"