[w33ble]

I was on Tmo's pre-paid plans for years. You can't beat their price. Their coverage left something to be desired though. If you're in city (for me, Chicago and later Phoenix), it works great. Once you leave that city though (ie, 15 minutes outside of Phoenix), you have no coverage. And of course, if you have an iPhone (or any AT&T phone), you're only going to get EDGE speeds.

More recently, I switched to Straight Talk. $45/mo (plus tax), unlimited everything (warning unofficial 2GB data cap). They are an AT&T MVNO, so you get the same network at a reduced cost. Now when I leave the city, I still have coverage, and it's usually very good anywhere I go. The only catch is that you don't get 4G coverage, which if you're on an iPhone, doesn't matter.

Of note, if you are on an iPhone, it will still activate on Straight Talk without any hassle. Setting the proper carrier settings for data and sms is another story though, and requires either jailbreaking or doing a swap trick with a Tmo SIM (you can get a pre-paid SIM for $4 though).