I'm on Simple Mobile's $50 4G plan after porting away from Sprint. I've never done T-Mobile before but it seems to use the same network and when you setup the APN on your phone, it points to T-Mobile in one of the parameters.
I might be a happier camper than most since Sprint is an utter piece of shit in NYC, but web access on Simple is very fast. I never miss a call and text messaging just works. And if you use a rooted phone, you can tether or do wifi hotspot to your heart's content for free, even though Simple states on their site that it is not allowed.
Sprint's current 3G network is sh*t in a wide array of places, including where I live in suburban NJ. The reason the majority of Sprint users and so excited about sprint is because of the plans to fix their horrible network, not because the current network is good. Obviously omitting places that already have LTE and network vision.