It offers plenty of fun, but it isn't a game for perfectionists.
1. A game that anyone can play and enjoy.
2. A way for those who are not skilled at playing instruments to express themselves through music.
So did they achieve it? I'll have a look at the two points individually.
1. It is definitely not a very complicated game to play. You mostly just use four different stiles. Piano stile (drumming with the remote and nunchuck), violin stile (Press B and hold for a tune and move the remote as though you are playing a violin), guitar stile (using the remote to play) and flute (simply press and hold the 1 or the 2 button). In addition to that you can press A or B on some instruments to change how they sound, and by shaking the nunchuck, you can make a little wah-wah on the guitars. So I do belive it is a game that everyone can play with no problem. The tune is already there when you start, so you just got to follow the rhythm. Frankly, there are some problems when playing it. If you miss a tone by just a little hundred of a second, it will sound aweful, so it's not quite easy for the youngest kids.
Can everyone enjoy it? Well, it's not appreciated by the hardcore gamer demographic. The lack of challenge is probably what they dislike. The game doesn't even give you point for your performance. The three mini-games are the only thing that measures your skill, and they are fun, especially being the maestro of an orchestra, but it's not enough. The only thing they've done to include the hardcore gamers, is to offer some nostalgia with game tracks to play. If you're a fan of Nintendo, you'll appreciate playing F-Zero or Legend of Zelda, but other hardcore gamers won't have much to look forward to.
2. "A way to express yourself through music." When reading that, wouldn't it be natural to assume that you could make music in the game? Well you can't, even though I don't doubt it would be possible for Nintendo to make such an option with a game like Wii Music. It would have been great if (with some work) you could play any song whatsoever, even songs you have made yourself. Nintendo even said that wii music was to be considered more of an instrument than a game. On what instrument can you only play 50 songs?
The songs won't appeal to hardcore gamers either. Because at that point (as well), Nintendo have gone with the "E for everyone" approach. There is next to no rock and no metal. Most of the songs are child songs or songs that everyone has heard. The first 8 are "clasical" such as "Swan Lake" or "Eine kleine nachtmusik" followed by no less than 22 "traditional" like "happy birthday to you" or "the flea waltz." Then there are 13 "popular" like "Every Breath You Take" or "Woman" and 7 video games songs including the main theme of wii music itself. If the track list is not to your taste, it's a bad chance you will enjoy the game. If you want the complete list, you could check it out on Wikipedia or something.
So what did Nintendo mean by saying you could express yourself? Well you have a lot of freedom when it comes to how you play the game, and that's the big strength of wii music. You can play any of the eleven music stiles they recommend, or make your own stile. You can use any of the 65 instruments (that is when "cheerleader" and "Cat suit" are considered instruments) and record over what you've done. So you can play single player and still play all six members of the band using your Mii. Besides using whatever instrument you like, you can speed up or slow down the tempo, and you can also make a slightly different rhythm. You can experience with the music, like turning F-Zero into a funeral march or Bridal Chorus into a rock song. It feels great to affect the music in the way you like, and for those who, like me, have unskilled fingers when it comes to instruments, it feels really great to play all instruments in a band then watch a music video of it.
So the game offers lot of fun, but also has a lot of disappointments. Apart from the ones already mention, I'd like to mention that there is no microphone, so you can't sing. I goes since Nintendo couldn't make it easier to sing, they wouldn't have that included, which is rather sad. The ridiculous thing they used to replace it, was an instrument called "singer" using flute stile. The singer don't sing any words, and it sounds more like a rabbit from Rayman Raving Rabbids than a singer.
Despite a lot of flaws, Wii Music is a good, charming and fun game. I don't regret buying it, and if Nintendo should make a sequel where you could make music as well, I wouldn't hesitate to buy it.