Last month my wife dropped her old phone and it broke, and I got her a Nexus 4 for $299 and we signed up for simplemobile (which is a Tmobile MVNO) for about $90 a month for the both of us.
Our previous bill from Tmo was $130 a month, a full $40 more AND on contract. Since it took a few days to wait for the Nexus she was asking "Why don't we just go to the store and get the free phones instead? Why spend all this money on a fancy phone?"
I'm glad tmo is doing this, but I think they will need to market it in such a way that it is obvious to consumers that they will save money in the long run.