The wii controllers must be placed inside 'tunnels' in the dumbbells. This creates three small issues, which aren't very bad but slightly annoying: a. the controller has a harder time communicating directly with the Wii because of said tunnel and the position you'd generally hold a dumbbell in isn't directly straight forward, so you miss points/hits/whatever. and b. by holding the dumbbell you're actually pressing buttons on the controller - the grip is removed so you've got direct palm to button pressure. and c. it's hard to disconnect the nunchuck from the controller which is required for several other games which dumbbells would make a better workout for, so I end up popping the controller out disconnecting. well, it takes up valuable time.
nice idea though and I'll continue to use them. the design could have been better.