Incredibly useful. I just happened across it when looking for a way to implement leaderboards and achievements on Windows Phone for parity with other platforms, but it goes way beyond that. Cloud storage, social authentication (along with a huge list of platforms like Steam and Kongregate), cloud based game logic and true multi-platform play (persist virtual currencies across platforms, etc). Definitely deserves more publicity than it seems to have. I only found it by searching for features.
3-2-1-Zombies!