Zelda BoTW is out, and the gamepad and button layout weren't doing this game justice so I took the plunge on this adaptor so I could use my Xbox One Elite Controller. In short, it's working perfectly (including paddles). You do need to upgrade the firmware on the adaptor, so to maybe save someone a little time setting this up here's what I did:
Update the firmware
Go here: http://www.brookaccessory.com/download/Wii%20U/
Download the latest firmware for XBOX 360/XBOX ONE to Wii U Pro Edition (the green download button)
(current version when I did it is 1.11)
Download the manual on how to flash the firmware (the red download button)
Follow the instructions in the manual to update the adaptors firmware.
(the tickbox for Controller Compatibility Update ensures newer Xbox One controller revisions work)
Pairing your xbox pad to the Wii U
Switch on your WiiU
Plug the adaptor into the WiiU (the adaptor lights up)
Plug your Xbox One pad into the adaptor (the controllers start button should light up)
On the WiiU gamepad press the Home button, then open the Controller Settings
Select 'Pair' in the Controller Settings
On your Xbox controller, hold DPad-Left, DPad-Down, A, X, and Left Bumper all simultaneously until the adaptor starts blinking
Almost immediately my Xbox pad paired successfully with my WiiU.
Other Brook adaptors allow button remapping, but I couldn't get this to work with the WiiU adaptor (maybe it isn't supported yet in the firmware). So I set up a profile on my Xbox for my Elite controller that changes the button layout to my preference for WiiU gaming and assigned it to the controller and it works great.
Hope this helps someone, and great service from Arcade World.