Got a Wii U over Thanksigiving. Played 3D World like crazy with the brothers. Beat it, but not 100% yet.
I think it's right up there with Galaxy/64. It started a bit slow IMO, but from about World 4 onwards I think it has the best level design of any Mario game ever. I do like exploring and stuff, but the level design trumped all of that for me.
Side note. We also downloaded the demo for W101. It completely threw me for a loop (in a good way.)
From reviews and thoughts about the game, I was expecting some difficult or obtuse controls.
The controls were simple. I really cannot comprehend how anyone would have trouble with the controls. If drawing on the gamepad was annoying, just use the right analog stick (that's what we did). Then the zR and zL (or whatever they're called) were used for the shield thing and dodging. It felt easy. I was mystified at how people could have extreme trouble with it, to be honest. Me and my brother did the 2 player mission and in the waiting area, we literally messed around for 2 minutes or less and figured most everything out. (The gamepad showed which shapes did what). Unless the game gets insanely complicated later on, I don't really see it. The non-major enemies were kind of small though and hard to see sometimes.
Anyhoo, bring on Mario Kart.