I have used my Pre + on T-Mobile for over 1.5 years, and just bought a Pre 3 that Im using on that very same account.
It works just fine - I believe the Edge radio is of higher quality, or maybe the antennae is better designed, because, Edge is fast (when you first set up the phone, you havent had the opportunity to wet up a wifi connection, so, when the profiler starts downloading apps and your specific information, it appeared to be faster than on my Pre+, is Edge only, and that showed 4 bars, when, in the past, Id have only 1 or 2 bars for edge).
Calls and MMS work just fine as well, so far. Its an amazing device, so far. The real advantage of this device on T-Mobile is that it does work seamlessly after you insert your sim card, as long as the device has been unlocked.
I am sure that one of the first patches the Homebrew people will whip up is a way to ditch the ATT screen that pops up on start up...
PS.. if, by some miracle (and, lets hope that this doesnt happen, for the sake of all mobile consumers in the US), ATT does wind up purchasing T-Mobile, there's a good chance that all T-Mobile customers would have access to ATT's 3G bands, as well.