Much more fun than I expected!
As soon as I started playing it, I thought it would be an easy game and rather silly, but as I advance in the game it started getting more difficult, more interesting.
You go through several worlds, each one following a particular theme where you are supposed to use one of your characteristics more. With each world you pass you learn more and more, so that you can pass on to the next, more challenging world.
While going through this worlds, you save toads and collect coins, puzzle pieces, music notes and mini-games. The music notes unlock songs for Peach's band, the puzzle pieces form playable puzzles and the coins ables you to buy things at the store, from more songs ans puzzle pieces, to life and power extensions, new powers and higher levels for your mini-games. With this, the game proved richer than I expected.
The DS properties such as the touch screen and the mic are well explored, both in the game and the mini-games. It also supports the Nintendo DS Rumble Pack, so if you have it, it's another good point to take into account.
In addition, the game is highly addictive, once you start it you really need to make an effort to stop playing it.
To sum up, if you are thinking about getting it, don't give it a second thought!