I'm running my unlocked ATT NL920 on T-Mobile for the last month. Works pretty good, I'm getting good speeds in the areas that I have good coverage. Problem is, for me, the coverage the T-Mobile coverage is not near as good as ATT. That said, I am paying $40/month less for a superior plan. Also, the international roaming aspects T-Mobile is offering are going to save me big money.
Note, that I can't get the wifi tethering going--which is a very common complaint elsewhere on this board from other ATT 920 migrants. Not that big a deal for me, as I am going to upgrade to a Lumia phablet once T-Mobile gets one.