Horrible.
The headline abstract pretty much sums it up. Although a music game is indeed a great way to get the entire family to play together, Wii Music is not.
Playing Wii Music consists of either flicking the remote and Nunchuk or pushing any button of your choice, depending on the instrument selected. The idea is to pantomime the instrument, playing a note whenever you feel like it. There is no scoring, grading, or any other sort of evaluation.
If you time closely to your part in the song, you get something resembling the song you selected. But there's nothing preventing players from playing off the beat or when that part really shouldn't be playing. The end result is something that resembles "Twinkle, Twinkle" at best, and a collection of barnyard noises at the worst. Heck, some of the "instruments" aren't really instruments. Really? A dog suit? I cringed as I played with (and I use the term loosely) someone who selected what I can only describe as cheesy karate screams as the lead melody instrument for Super Mario Bros. Gag.
Making a simple music game anyone can play isn't a horrible idea, but the gamer(s) of the family will refuse to play this abomination. I admit that I did not check out the drum trainer mode due to a lack of motivation, but the main mode of Wii Music is the worst gaming experience I've had at any E3. I never would have predicted a Nintendo-developed title could hold that dubious honor. Rock Band and Guitar Hero do a fine job of engaging the whole family; I only hope retail sales representatives across the country can convey this to would-be consumers.