My boyfriend got a Wii from his parents for Christmas and we have so much fun playing with it. The gimmic is that instead of using a typical controller, you use the "Wii-mote" with which you have to actually move like you want your characters to move instead of just pushing a button.
My favorite game to play on the Wii is Wii Sports, especially the bowling game. In this one, you have to grab the controller and pull it back over your shoulder before flinging it forward like you would a bowling ball. I'm no good at bowling at a real alley, but I somehow always get strikes at this game.
One annoying thing about the Wii is that you have to put a sort of reciever on top of your TV and it is pretty spindly. Ours has fallen on the floor a couple of times, but thankfully hasn't been stepped on. Also, you have to make sure your wrist strap is on tight or your controller could go flying across the room if you get really into one of the games. Nintendo actually started selling them with rubber cases on them because of all the people breaking stuff in their living rooms.
Everyone I know who has a Wii loves it. Too bad they are still so hard to find. The graphics and sound are very good. New games are coming out all the time.