Own one, had it since last X mas and very happy with the results.

After playing it I became increasingly aware of the correlation between me skipping a meal, and me losing weight. I now have 3 meals a day every day, sometimes I used to skip 1. And now I've gained a couple of kilograms :) I;m now very close to ideal BMI - I'm still slightly underweight

I think the accountability is the most important part.  I can be walking around the street slouching, and then that little bastard will be telling me my center of balance is way behind my center, and I'll stand up straight and fix my posture.  That alone has done wonders for my lower back.

@OP, I've been using Wii Fit to gain weight, and have been doing so very very slowly.  It makes me actually think about my weight, posture, diet, and exercise routine.  I used to just go on exercise binges every few months, and go back and forth between being really strong and really lazy.  I still kinda do that, but to a lesser extent, but my posture is better and I weigh a little more.  This was cheaper than 1 or 2 months at a gym, so I'm very happy with the results.  And I just feel healthier.