I've used it about a dozen times, and the call quality has been decent. There's a bit more latency than normal voice calls, but nothing too terrible, and I'm sure some of that was spotty mobile data service.

In terms of wifi tethering on T-mo, I'm not sure how that'll go, but it seems pretty promising. The deal with Nexus phones on Sprint was that it was officially unsupported / prohibited, but there was nothing they could do to stop it since it was pure Android without any carrier limitations.

From what little I've read about tethering on T-Mo, Nexus phones or other phones with third party roms loaded seem to do tethering okay, but there are some people who claim that T-Mo detects tethering by examining user-agent strings, which is pretty vile if you ask me, but also easy to defeat by either changing your browser's UA string or routing your traffic over a proxy or encrypted tunnel.

I'll certainly be testing it out when I travel next month to avoid hotel internet costs, but even if I lost tethering, it'd be worth it to cut my bill by more than half.