I think since E3 2008 most people assumed that Wii Music would be getting mixed reviews. Considering the information released about Wii Music stated it wasn't an actual video game. But a toy. From what I've been reading so far, I'm actually pleased it's doing this well. The general opinion is that while Wii Music is fun in doses, it gets stale and repetitive after awhile. Of course I don't have to tell you, because here are some of the reviews from around the way.

IGN - 5.0/10: “I wouldn't qualify Wii Music as an abysmal failure. The truth is, I like some of the concepts powering the video game. The ability to dynamically alter music using a variety of instruments. The fact that you can layer together different songs and really create your own style. And the integration of Mii avatars, not to mention WiiConnect24 support, definitely add further polish to the fun and simple presentation. I think for all of the above reasons, kids may really latch onto Nintendo's latest effort (although I feel a little sorry for parents who must endure the cacophony of noises coming from their child's bedroom).”
1UP - A: “Wii Music may have a hard time winning over the skeptics who just want to laugh at it, but give the video game the chance it deserves. You just might realize it's pretty damn fun being in on the joke.”
Gamespy - 3.5/5: “The video game also makes excellent use of your Mii collection and beyond playin as your virtual avatar, you'll see a wide selection of Miis crop up in the orchestra and mini-video games, which adds a level of personalization to the proceedings. Wii Music is an odd duck. It is far from a traditional videogame, and more akin to a tech demo or social audio experiment. The fact that it is so very basic means it's completely accessible to anyone, but at the same time there's not much here to interest regular and core gamers.”
Cheat Code Central - 3.1/5: “Overall, other player’s feelings towards Wii Music will largely depend on what they believe constitutes a modern videogame. If you believe video games are non-competitive ways to express yourself and where not much effort is required, then you will probably love Wii Music. However, if you believe video games are all about skill, scores, and hard work, then you probably won’t enjoy this title at all. But either way, it can definitely be said that Wii Music is a unique title that stands out among its peers, for better or worse.”