Have many people really lost meaningful weight using  fitness video games?  Most of the ones I've tried like Wii fit really didn't seem to do much at all. I'd be curious to see if there's been any unbiased studies done on the topic.

The only one I can ever remember playing and feeling like I ever got an actual workout was the DDR series. And perhaps coincidentally (or not?) it's also the only one in my experience that was remotely fun to play.
 
It seems to me there is low hanging fruit here for an aspiring fitness genre developer for a concept that actually seems to produce results and that's the P90x franchise/craze. It's certainly the regimen du jour (like Tae Bo and such before it) and it's heavily promoted on all the ESPN properties (guess they are done using nutrisystem).  Ancedotally it's the only one of those sorts of regimens I've actually seen work for friends and family. That stuff can get you seriously ripped. 
 
but even if it didn't it still has enough mindshare that I could see the brand being really valuable for a fitness game.
 
Furthermore they have a version called "Insanity" that doesn't require any equipment. Seems to me it would be pretty easy to adopt the workout vids into a connected KInect game to enhance the experience.
 
the reason I could see people actually going for a Kinect game version is that the P90x workouts are really really hard and you are allowed  by the workout to stop anytime you are overwhelmed (in fact in the videos I've seen many of the example instructor minions regularly collapse from exhaustion). So I could see how it could be useful to have something like Kinect keep track of how much you actually do from day to day.  And possibly make it more fun to do. I know if I tried it I'd need some sort of reinforcement to keep me going.
 
I still can't fathom why no one has come out with an honest to goodness good fitness game for the Wii or Kinect yet. The fact people so many copies of that Wii Fit nonsense should developers there is a market for it.  A good portion of the original console gamer generation like myself are aging to a point where our metabolisms are slowing and in general have considerably less time for gaming with no side benefit. And hey America in general is really fat, and constantly spending money on garbage diets and such to lose weight. So there's cash money to be had here for developers.