Nicely played. Rollover data is something that customers will only use on occasion, but it's a concept that people can easily understand. It directly responds to other carriers' lowering the cost of their individual plans by offering up something that AT&T and Verizon would be loathe to offer themselves. If AT&T and Verizon match this move, they would carve a huge chunk out of what they currently collect from overage fees. T-Mobile already did away with overage charges, so this move has minimal financial impact on them, but huge marketing upside.
Smart strategic move.
Just in the last year as a T-Mobile customer, they've added free tethering, international roaming, and unlimited music streaming -- and I'm on a $30 prepaid plan. And the service in my area has gone from HSPA+ to LTE to wideband LTE in less than 12 months. As long as these "Uncarrier" moves and network improvements keep adding value to my service, I have no reason to even consider other carriers.