Here is an interesting article about Wii Fit and losing weight. It points out the flaw with the Wii Fit being an effective exercise tool - people are still going to have to put in the work to see the results.

As with most exercise equipment and programs, people tend to play Wii Fit for a few weeks and love it. Then they don’t play as often which then leads to not playing at all. The real culprit to Wii Fit being a long-term exercise solution is monotony. The games are the same each and every time you play. It’s the same moves and once you’ve played it a few times, you naturally lose interest. Also from an exercise standpoint, it’s not really good to do the same thing day in and day out as your body will become used to it. You really need to turn it up a notch to really burn the calories.
One thing I would love to see for Wii Fit is downloadable content. It would be great if through the Wii Shop you could download new fitness games or even Yoga routines that put all the moves together. Also a multi-player mode would be awesome as then you could work out with a buddy either online or in person.
I think Wii Fit is just the beginning when it comes to exercise games. There will be plenty more coming out that will make use of the balance board and hopefully will learn and improve upon the shortcomings of the Wii Fit.