Shiny new board and "fitness" work do not a great game make.
To start off positive, we'll look at what's good about Wii Fit. It certainly is innovative, and you'll never have played anything like it. Some of the games, particularly the ones associated with balance, can be enjoyable, and many of the games are unique, some even playing like old-school games with a twist. If indeed it can help with fitness, that will be good too.
However, there are some negatives. The most blinkeringly obvious is the price. At £70 (to be honest, I don't know the American price, but it will probably be pretty steep), it's about twice the cost of a normal Wii game. The content of the game itself seems to be lacking in any real substance. It has a Brain Training-style body age calculator, but this is so random the Wii Board's monitoring use doesn't extend past scales and basic calculations. Once the gimmick wears off, Wii Fit will not be a great experience for multiplayer, unlike the superior Wii Sports, and so has been marketed by a single-player experience. However, there is a problem with this. What is there to keep the player coming back. There's an assortment of games, none of which are good enough to keep you coming back in themselves. One of the major draws should have been body age, but that is so dependent on your talents for that particular game that you may as well judge by how well someone can play Super Mario Galaxy. And I can weigh myself without an irksomely squeaky little board, thanks very much. Also, its value for fitness is highly questionable, and you'll get less out of Wii Fit than spending a quarter of the time doing real exercise.
In conclusion, Wii Fit is misguided. Its design means it can only ever be a single-player experience, but it can't do that well enough. I wouldn't recommend purchasing this.