Amazing game. I'm only 40 stars in (probably out of 120) but I've already seen so much variety and faced so much challenge. And it's the first time I've actually been awestruck about the music in a Mario game (fully orchestrated with some particularly excellent new songs).
It's receiving unbelievably positive critical appraise. 10/10 at multiple websites. See the full lineup of reviews here.
If you have a Wii and haven't played this yet... what are you waiting for?