I got a Wii U a few weeks ago, but haven't used it too much yet.
Just been slowly working my way through Mario Kart 8. It didn't wow me at first, but the controls are a lot tighter than in previous iterations and I really like almost all of the tracks. That said, it is disappointingly easy to get 3 stars on each cup now. You used to have to have to beat second place by a fair amount and use minimal items, but the only requirement now is that you come first all four times. Small niggle though.
I've yet to get online, but I've heard it's pretty awesome.
I upgraded my Wii VC version of Earthbound so I can play it for the first time on the GamePad