I think that Wii Music came out maybe too early. There are better forms of MIDI on the horizon that sound a lot better and more like instruments than they do in Wii Music.
Also, what's even more scary with Wii Music, and why most people give it a bad rap is that it quite literally requires the person to be creative with their music playing. It's super easy to play instruments badly, and this very much holds true in Wii Music. However, when you can actually be creative, follow the rhythms and patterns of the song you're playing, then you can get something great out of it.
Compare that to Rock Band where you don't have to be creative usually as you play what they tell you to play. What's great is that you not only feel more like you are in that band (AC/DC or whatever), but you can add your own little flavor with things like the small Drum Breaks.
Personally, I'm not a fan of Wii Music. I hate being the person to say this, but when I play video games, I want to feel like I'm freaking awesome. Whether it's saving the galaxy, or being a rockstar. I don't want the game to make me realize how musically challenged I am. Thus, if I'm going to go through all of that trouble, I may as well pick up a real instrument and spend my time working on that. Yes, it's not as easy as button pressing, but you'll get the same effect out of it. I think that feeling of "I want to go get an instrument!" is what Wii Music is also trying to invoke, to get people interested in music. Thus, if the goal is to get people at least interested, then Wii Music has already succeeded.