my friend has this game, and she plays it a lot, meaning that she likes it.
it does give you an exercise if you do it right, and follow the exact instructions. she came to school with her arms and legs sore because she played Wii Fit all day
so yes, i think it does give you a good workout. i havent tried it myself, but based on her saying how awesome it is 24 hours a day xD i'd say it works pretty darn well. (: