A demo for Rayman Legends is now available on the Wii U's online store. To download it for yourself, click the shopping bag icon on your Wii U menu and you should see it appear “in the spotlight”. If you can't see it there, scroll down a little and you'll see a “demos” icon. Click that and Rayman Legends should be listed (I can only see that and Fifa 13 on mine at the moment). Or you can just use the search to find the demo that way.
It's definitely worth a download. Rayman Legends is one of the best titles coming up for the Wii U, and it's a real shame it wasn't available at launch. The demo consists of three levels, including the awesome “Castle Rock” level that syncs Rayman's fast-paced movements with the beats in a rock song. The whole thing is playable with up to four other people, with one person playing on the GamePad and the rest using Wii Remotes.
The GamePad player takes control of Murphy, and uses the touch screen to manipulate the level to make things easier on the other players. It feels much more involved than the “Boost Mode” used in New Super Mario Bros. U, and if you're only playing with two players the one on the GamePad can switch between Rayman and Murphy to get the best of both worlds.
Download the demo now to get 30 opportunities to play these three levels. The game itself comes out in February next year.