My nephew has got it and him, his brothers and their sister all love it. I've yet to try it though. I'm doing a bit of babysitting later so I'll see if I can get a go! If I do, I'll report back.
They've also got Wario Ware Smooth Moves, which I've played (and think is fairly similar) - I borrowed it when a couple of my female friends came 'round. They enjoyed it but prefered Guitar Hero (which they brought 'round!)
OT - Guitar Hero (or Band Hero?) should be making it's way to the Wii in the future.