My question is, how does it know what you're playing on Wii?
I guess you must specify what you've been playing on a given time for the service to know, unlike Steam or the XBOX 360.
The idea is not new, there are a few other similar services, each one with its own take on how your gaming habits should be tracked (including one from Napster's own Shawn Fanning, which EA bought a while ago), but AFAIK all of them have flying under the radar. Raptr seems to be the most inclusive one, and most important, the one who takes the biggest advantage of the different information provided by the various systems it can track and that's some time you save from having to input the data manually. There is a good review
by the folks at Ars Technica, BTW.
It's also nice it has a library of games, with info, pictures and videos of the games ala MobyGames, BUT it has a serious lack of retro systems, which is quite disappointing for me (that, and I'm too lazy to migrate data from one service to another :P)