I'm definitely looking forward to it.. I don't play on XBL much and because of that whatever rank I had with a particular games dwindles to nothing during the periods that I don't play and I end up playing against people far below my skill level because of it.
Also I'll try a new genre of game soon after it's release and find myself in way over my head because while the game is new to everyone, there are lots of people familiar with the genre and should be ranked higher right off the bat.
Based on this as well as what MS has said at various conferences it soulds like they'll track your rank within individual games, genres, and overall...
Sounds great to me