Mario Kart Wii is great! My GT5 Prologue has been collecting dust since I got MK. Online play is so much smoother than GT5p too.
BTW, I have had the Wii Fit since it launched in Jap last year! It certainly works and if you follow the exercise and don't cheat, you actually burn quite a lot of energy. The football game in there is very additive, everyone's been coming to my place trying to beat the top score. But you just keep thinking, how does Nintendo come up with these clever things!