Picked up my copy of Wii Music today, and went through all 15 lessons for the drum set tonight. Some of the lessons were more difficult than I expected, but I think I'll get all of them perfect with a bit more practice. Also, I was having trouble with the hi-hat pedal (using the balance board); keeping the hi-hat closed was fine, but whenever I would try to keep it open, it would often close unexpectedly when I stepped on the bass drum pedal. Not sure yet if that is a calibration problem or if I just need to fix my technique.
After only about an hour of playing so far, I can say it definitely seems more fun than I had expected. (I've always been looking forward to the drum set mode, but had become a bit apprehensive about the other modes recently.) I was a bit surpised at how fun it can be to play keyboard-type instruments (including marimba, etc.).
In some ways, I think Wii Music is going to prove more difficult than Guitar Hero, et. al. (Matt's video on IGN is evidence of that, I believe.)
Having said that, I can certainly understand why Wii Music doesn't appeal to fans of GH and RB. They are very different approaches to music games.