I just made the switch about a month ago after doing their 7day test drive. The LTE speeds destroy att, the international coverage is unmatched, and the prices are only rivaled by sprint.
HOWEVER, indoor coverage absolutely sucks. In downtown Seattle you'll struggle for coverage inside buildings and in restaurants. If you have wifi available, you're fine because of Tmo's wifi calling, but if you don't, you're disconnected. This is something I'm willing to put up with for the time being as they will inevitably improve.