All about fun.
I will be buying a Wii, simply for the same reason I bought a cube and a N64 before that and a Super Nintendo before that.
Innovation is always a winner for me.
If you remember when you first saw:
How great "mode 7" graphics were
Seeing 4 way split of a multiplayer game e.g. Goldeneye
Getting amongst 29 other cars in F-Zero X
Seeing Mario 64 on display
Then I think you will probably understand where I am coming from.
I have a PC for your run of the mill games, your pro evos, call of dutys or toca race drivers if you will.
I "plan" to have a Wii (hehe) for those drunken nights in with friends.
And (esp for the Hmmm poster) I will carry on playing Hockey at weekend for my aspect of real life interaction (I also work aswell - if that counts...).
So the motion controller will get old after a while. But if it gives me as much fun as playing: Bomberman 2, Super Mario Kart (Time trial was brilliant for deciding who gets the beers in), Goldeneye, Monkey Ball, Smash Mario Football to name a few. It will be worth the money.