I recently purchased an unlocked international Lumia 920--I live in Austin, which was recently re-farmed on the Tmo network--could not get better than Edge or 3G speeds. I returned that phone for a Rogers brand unlocked L920, put my Tmo sim card in and instantly had 4G speeds and have had zero problems so far. I had to do some manual settings for MMS but once done, that has been great too.
If you are on Tmo and really want the Lumia 920, the Rogers unlocked unit is the way to go. I have heard that Tmo is supposedly getting a top shelf Lumia later this year, but having been a Tmo customer for years, I will believe that when I see it! And they certainly don't seem especially interested in anything but Samsung or finally getting the iPhone. The HTC 8X I bought from them didn't even offer wireless charging for the unit because it was an "exclusive" feature on another carrier--just depressing, really. I am so very glad I was able to switch to a Lumia 920, aside from 8X being a fine phone.