I've been blown away by the quality of this game.
My impressions so far:
1. FAR more challenging than the first one. This is the first time in a VERY long time that I've actually gotten really really really stuck on some levels.
2. Bowser is too easy, as always, but this is OK since other aspects of the game want to make you through your controller through the tv screen.
3. I didn't think it was possible, but the music in this game is even better than the first one (which also had an epic soundtrack). I would seriously play the game JUST because the soundtrack is so good.
4. Best graphics on the Wii, period. Really beautiful game, with a lot of love put into it.
5. Did I mention that it's really hard?
6. Pretty much, this is Galaxy 1 mixed with 64 AND NSMB wii and then on top of that, perfected in every way. The level designs are fantastic. There are some really weird, and fun situations with gravity (some good ones in some of the bowser levels).
It is DEFINITELY worth getting this game.