In your mind you know you're not covered in sensors and that the motion is controlled through the handsets, but this doesn't stop you going through all the motions like it's the real thing. When you bowl you crouch as you swing. Playing tennis you twist your torso to get some power in the ball. As you box you're jabbing like a pro. It may not be real exercise, but at least you're active and not sitting down twiddling your fingers and thumbs.
It's also good to play solo or with family/friends as it's incredibly intuitive and you don't need to know all the special button combinations. We've had everyone from kids to grandparents up.
The only warning is to make sure all the players have the handset strap is round their wrist. It's easy to see how they can be thrown at the TV creating a very costly mistake.
All in all, while some of the graphics aren't great, you do get lost in the games. I'd definitely recommend this.