Personally im 3 weeks into using the wii fit and im finding it amazing, except for today (went for another massive meal for a '6 months' present from my boyfriend XD) ive gone on it everyday.
Its perfect for me as im very very self conscious and have appalling image issues. I was bullied in gym and pe at school, especially in the changing rooms for the last 2 years of school I became very depressed and refused to do any exercise. I ballooned and it knocked me so bad. I didnt want to go to a gym as I dont want to be judged, so wii fit works very well.
Its not a serious 'you will loose weight now' type of game. Its very soft and almost fluffy. However its so good for tracking what you do, it tracks your weight and you can create an activity log of things you have done outside the wii fit. Its a very good stepping stone prior to going to a gym, it gets you thinking more and makes you feel guilty - in a good way.
If i go up a pound I go jogging the next day - which I have never considered before.
Basically its an alright game
but a brilliant motivator!