Its awesome, I got mine the day it came out and been doing it every damn day !
When I went to the gym, I never went everyday, so thats an improvement right there.
The more time you spend on your Wii Fit, the more repetitions and new exercises you'll unlock. I mainly wanna be fit, not loose weight but it's really fun. So I get all the excitement around it.
As someone else said, it's lacking a couple things. Like, I would really like to be able to select a part of my body that I wanna work out and punch in the time I wanna spend or have the machine select an x ammount of execises to do. Then you could switch body parts every day or something like that. That would be the best 'cause now, you have to select one exercise at a time and it just takes too long imo.
Still, when you do it a lot, it becomes quite difficult and I already can see that im fitter than before.
Gotta LOVE my Wii Fit !