I have only had a few brief goes of the Wii, but it is a very clever advance in interface for gaming in general which Nintendo are great at. Have a look at this video for Alone in the Dark (I still remember first playing this revolutionary title on Windows all those moons ago).
Wii Fit is the next clever extension for the Wii and there are great reports of people using it to actually exercise and lose weight etc. Adding the balance pad to the exiting controls and a host of 'games' and exercises for people - this is a significant improvement on the old exercise video. It is interactive, changes to suit your requirements and offers a great deal of variety with feedback.
This is one of the official trailers:
And there are plenty of other vids on youtube of people playing - interestingly some now net famous ones of girls. havent found any good data of whether it is actually helpful, any trials or studies with data. No doubt some will come in the near future...