This is an absolute cracker of a game. The best Wii game ever and unrivaled.
The Subspace Emissary: This is really a game in a game that is quite addicting and will last about 10 hours. Try to save the world from Subspace as the Nintendo characters join together to save Subspace against the evil Tabu. This is the main reason why I gave this game 10/10.
Online Multiplayer: Although many people rate this area reasonably low, I believe it still is fantastic although there can be some improvements. Nevertheless it's even more enjoyable to think that your playing a human player. There are more features once you have a friend to play with, but however, you have to rely on a different source to get friends.
The rest: There is great replaybility and there is a good chance you can get 40 hours plus. The many features to do once you've completed the Subspace Emissary will not make it boring. It is a fun part and you can unlock more and do many other things.