Does the old Wii sensor bar fit the new Wii U?
Yes, both sensor bars are identical and will work on with the Wii or Wii U console. The Wii sensor is a very simple device. It's not actually a sensor. It is best described as an Infrared transmitter, as it does nothing more than create 2 dots of Infrared light. The sensor, the complicated part is in the Wii RemotePlus
How much does a replacement GamePad cost?
Unfortunately you can't buy an extra GamePad for the Wii U. You can however order a replacement GamePad from Nintendo, if you happen to break your own one.
How many GamePad controllers does the Wii U support
If you take your Wii U controller to your friends house, can you both play together with 2 controllers? The simple answer to that is no.
However, Nintendo did confirm that the Wii U can technically support 2 GamePad controllers, albeit with a performance hit. Connecting 2 controllers would halve the frame rate on each screen to 30fps. This would have to be enabled by the software and no developers have thought it worth doing so far.
Can I take play on the Wii U GamePad away from the console?
No you cannot play with the GamePad away from the Wii U console. The Wii U controller could be considered to be a streaming device, as the Wii U console streams images to its screen. If the console is off, the GamePad will not function.
There are rumours of a future update that will enable the Wii U GamePad to play small games away from the console.
Do the old Wii remotes work with the new Wii U?
The rebranding of the new Wii U remotes only went as far as the packaging. The remote itself is identical to the old one, so yes you can use your old Wii remotes and RemotePlus units with the new Wii U.