Holy crap this is an awesome idea. I really hope the people who decided to make video games movement enabled was given a massive bonus. Ok first thing first for those of you who are thinking that this is just another gimmick you should look a little bit closer. The Wii Fit is a really well put together piece of work.
The first thing that struck me about the Fit is that it comes with batteries. That is just a godsend for someone like me who ALWAYS forgets that kind of crap. Out of the box everything is simple and easy to understand it took me about 2 minutes to set everything up and synchronize it. Then you take a few minutes to register your Mii character and you're away! It measures your balance, center of gravity, weight etc and spits out your "Fit Age" which is supposed to be an indicator of your overall health. Generally speaking this process was pretty good but a small problem is that it's partially calculated on your BMI which is not infallible. For example, it said that James was overweight when he is clearly not, the reason for this is that he has very heavy dense muscles on his legs from whole body vibration machine, more about this machines you can read at http://gadgetforhealth.com/. Anyway it's a good guide but I wouldn't rely on it for your entire health and fitness needs.
The games are well thought out and within about 20 minutes of playing I was sore in places I didn't know exsisted. I'm not a gym buff as you guys know but I am fairly fit. I used to walk 4 kms to and from the train station every day so I'm not really out of shape but this system actually tested me. The balance games are challenging but not so much so that they are frustrating. If you play for a while you unlock more and more things which is fun too.
I can't vouch for the muscle exercises but after James did them he was VERY sore indeed. The trainer is very helpful and she/he lets you know what to do in language that even a workout virgin could understand.
Overall it's a really helpful tool in managing weight and as long as you treat it as part of a bigger fitness program, as it was designed to be, you should find it as enjoyable as I did!
The only thing I can say is that people with thin skin should avoid use of this as if you are overweight and give excuses the games chastises you!