I got the Wii Fit for my birthday and I just gotta say it's awesome! It really does help with fitness and the best part is it's tons of fun!
It helps you keep track of your fitness progress by tracking your weight--yep, it weighs you! It saves all of your stats so each day when you use it (and it's so much fun you do want to use it daily) you can see your changes.
It's a great addition to the awesomeness that is the Wii!