One thing about a lot of the Wii fit exercises is that they don't last very long. You can change the number of times you do a lot of the yoga and fit board exercises, but for stuff like the jogging and sticking with the defaults, each exercise only lasts a minute or two. I can easily see many overweight fanboys buying this and then wondering why their weight hasn't gone down even though they cut down to only three buckets of KFC instead of four, did SIX push-ups, and jogged for a WHOPPING three minutes!
I do believe that Wii Fit can help you to lose weight. And I like the yoga stuff. I honestly felt good after doing some of the yoga exercises last night. But the game is set up with casual non-gamers in mind, and you can just load up the system, play a mini game or two, stop, and shut the system off. They should have added a more rigorous "forced" program.
BTW, you can full install the game on your Wii. It shows up as the "Wii Fit Channel."