In my opinion it is not meant to replace real gym workouts. It may get people started on the right track to try working out. Mine arrived yesterday and I used it for about 20 minutes (not including the time wasted in the setup if th Mii) and it is a interesting concept. Nintendo actually did a decent job on it. The BMI test called me obese (I'm not fat...I'm big boned) but who cares.

The balance games are interesting. Just a slight change in balance can affect your rating in each game. It can get tricky. Sure it's just a bunch mini-games but if you stop and think about it the game is having you stand/lean in specific positions for set lengths of time or doing repeated motions. This all works muscles holding the positions.