The Wii U uses all sorts of Wii accessories and plays most Wii games. Backwards compatibility done right, it seems.
However, the new addition to the ever-growing line-up of controllers that work with the Wii U, the Pro Controller, will not be backwards compatible with Wii games, nor the Wii console. The latter is understandable, the former is an annoyance.
Those of you looking to drop the Wii Classic Controller and instead use a Pro Controller, thus cutting down on the number of controllers you have to keep around to get the most out of the Wii U, you're shit out of luck.
To fully get all you can out of the Wii U controller-wise, you're going to end up with a Wii U GamePad, a Pro Controller, a Wii Remote, a Wii Nunchuck, a Classic Controller, and a Wii Fit. I'm probably forgetting one or two—or five.
On the bright side, the Pro Controller is rechargable, and lasts up to 80 hours on one charge. In comparison, the Wii U GamePad gets five hours out of a charge. Better get that Deluxe Set Wii U bundle that comes with the Wii U GamePad charging cradle, then.
Read my impressions about the Wii U Pro Controller and the rest of the Wii U hardware in my Wii U hardware preview.