From my limited experience playing the “game”, no, it doesn’t suck. Surprisingly enough, it’s actually a lot of fun. Obviously, it’s not a real game, and if you go into it expecting a Guitar Hero-esque challenge or something, then you’ll be sorely disappointed. Wii Music is about one thing and one thing only: having fun.
I don’t always love Shigeru Miyamoto’s games (though I do love most of them), but he has this way of making everything he touches just pure fun. The games aren’t necessarily flashy or epic in any way (especially this one), but it’s everything it needs to be and nothing more.
If you really give this game a chance and get into (and have friends or family to play with), you are guaranteed to have a lot of fun.