Holy cow! Are tmo's monthly 4G plans a typo or something? What the catch?
The catches are:
1) no roaming
2) ty/nonexistant customer service
3) prepaid (service shuts off if you don't pay by midnight of the 30th day)
4) no free nights/weekends/mobile-to-mobile minutes. Get what you get.
5) no conditional call forwarding
6) have to disable T-Mo voicemail to use Google Voice voicemail natively (see #5)
7) must bring your own device (aka full price $$$)
That being said, I use T-Mobile prepaid ($30 5gb plan) and its pretty solid. But my biggest warning is this:
know what you're getting into before signing up. The $30 plan is literally unbeatable if you don't need many minutes. The $60 and $70 plans can definitely be beaten by Straight Talk ($45) and Simple Mobile ($60 but more data). But if you just like going with the bigger company the $60 and $70 tiers are pretty solid. Knowing where the data throttle point is and knowing you can't be kicked off is good. Not all prepaid carriers offer the same benefit.
Obvious pro's (to any prepaid carrier):
1) PRICE
2) no contract
3) bring your own device
4) unblocked tethering
5) switch carriers/plans any time
6) price, again. prices are amazing.