It's actually not at all bad but, as ever, you only get into it what you put in. Its scoring system is iffy and easily cheated, so you have to stick to the spirit of wanting to get fitter.
So it's a good and convenient motivator with some amusing mini-games built in. I use it regularly. But it's not a miracle machine.
I actualy think it is
Also here's the Wii Fit thread: http://www.killermovies.com/forums/showthread.php?s=&threadid=482840