Just come out of a roughly 2 hour session with the game, of course it didn't seem like that long because I was enjoying the game so much. I recommend this game to anybody, you should at least give it a rental.
The thing that makes the game so great, is that it's so different from all the other core games out there, not just different for the sake of it, but different in a good way.
If I was to describe gaming as a three course meal, you would have your casual games as a starter, your serious games such as your Killzone's, Call of Duty's etc... as the main course and for a nice tasty dessert you'd have Super Mario Galaxy 2.
I'm about 20 stars in and just about to start world 3, although I still need to complete some galaxies from the previous 2 worlds.