Its not just a nunchuck in itself, it also comes packed in with a stand that will hold the Wii console, SD cards, the nunchuck, and it also appears to be capable to hold the Wii-mote behind the nunchuck.
It doesn't look very slick though. It looks kind of sloppy and cheaply made, but it is nice to see that metal will hold the nunchuck and Wii-mote instead of cheap plastic.
3rd party devices are usually cheaper than 1st party ones, but not in this case. The device will sell for $26.82 USD, the official Nintendo nunchuck is $19.99. I guess that, to be fair, you do get a stand to hold the Wii with it, so it isn't a bad deal. The only problem is that this nunchuck may not be as responsive as the Nintendo one, making this a bad buy.