As a product it's too pricey, in my opinion, to do well. You get two "Activity Monitors" with the cart, and you can buy them separately, but there nothing else here.
The adults who have a DS will buy carts like Brain Training and the language Coach but they'll probably think twice when it comes to extra gadgets and gimmicks. And very few kids are really going to want this. You could get two games for the same price.
Maybe if you combined the monitors with a good game, where your daily stats help you progress or give you bonuses within the game, then I think it would have a better chance. I bet the Japanese would buy into a gimmick like that even more readably that here in the West.