Only "little" problem with iPhone in T-Mobile (USA) network is that it works only as 2G (not 3G). That's the fact. Hence I resigned from "converting" one. I am loyal T-Mobile customer (due to more international spport and agreements than AT&T has which is required where you travel and call in GSM networks).
If iPhone worked in Wifi-only mode I could use my T-Mobile 4G Hotspot device just like for my Macbook Air and iPod Touch.