I have been looking at it as well, but:
1. T-Mobile coverage outside of major metro areas is non-existent in a lot of areas.
2. It actually is not cost effective for a lot of users as in order for T-Mobile to pay off your ETF you need to trade in your phone and then turn around and buy a new phone from them. So no contract, but if you leave T-Mobile then you owe whatever may still be due on the purchase of the phone.
3. In my case, it makes more sense to just pay the ETF for my on contract phones with AT&T and then keep the devices and use them on T-Mobile outside of this plan, which I would do if their coverage becomes substantially better. I can save money moving my family plan to T-Mobile on a monthly basis, but their coverage is so poor in many areas where we travel to that it's not worth it at this point.
Not quite the "uncarrier" deal I thought it would be, but at least it's getting AT&T and Verizon nervous to the point that I would imagine is causing them to rethink some of their policies and prices.