Wii Music is fun..... For about three minutes.
But I gave it a chance. I was wrong. It was much better than expected, but then it just got boring.
The main reason why it's boring is because there's nothing to do. The tutorial is probably the longest simultaneous sit-down you will ever have on this game.
It is a little fun to experiment with the instruments, though. They vary from boring to cool and to WTF (you'll see if you ever play the game).
You can make music videos, which is probably what you'll spend all of your time doing, since you can just freely mix some music.
The concept is simple and easy to get a hold of, but at the same time it gets to repetitive for that very reason.
To get the music right, it doesn't go like a Guitar Hero game or anything. Instead you just beat the Wii-Motes (or Wii Remotes, whatever you want to call them) in the correct beat and the song will turn out right. Or, of course, you can completely freestyle it and go crazy.
You can design an album cover for your record. It's not very detailed, so don't expect a lot of fun or customization with that.
This game is kinda fun if you're just messing around with a friend and want a good laugh, but if you were expecting a good music sim or rhythm game, you're out of luck. Check out Rock Band.