So I just came back from trying the Wii U!
I played The Wonderful 101, Mario and one Nintendo Land minigame.
The first thing to mention is the controller... It's actually very comfortable and surprisingly light. It fits in your hands perfectly and you can press all the buttons easily, with the touch screen being easy to access by letting go of the controller with your right hand. I did have minor issues at first, specifically with the face buttons being under the right analog stick (and their overall positioning being a bit weird, at first I kept pressing B or Y when I wanted to press A), but after playing it a bit I did realize that I'll get used to it pretty fast. Otherwise? No problem, this controller is really good. I also got to hold the Pro controller and it was nice as well.
Just a quick note on the console itself... It's basically a really long Wii. Other than it being almost the length of 2 Wiis (from what I could see, I couldn't touch it or anything), there's not much to say about it.
As for the games... The Wonderful 101 was really fun, it's basically an action game (kinda like DMC or Bayonetta) but from a really far-away perspective and a fixed camera angle. I didn't have time to figure out combos, but it was really solid and makes pretty fun use of the screen. Definitely getting this game as soon as it comes out... The funny part though is that the guy who was handling that particular "booth" (while I was playing, it was someone else right before) didn't actually know how to play the game or what the controls were, so I told him how the game was played as I was playing... Oh and the guy before him was calling the game "Powerful 100" for some reason... "Interesting" note, this was the E3 demo, as it still had the "Project P-100" title.
Mario... was Mario. There's really not much else to say, it does look pretty fun overall, but it doesn't seem to do much different from the other "New Super Mario Bros" games. The flying squirrel suit was fun to use.
Nintendo Land... My friends were playing the Luigi's Mansion minigame while I was playing Wonderful 101, and they told me afterwards that it was really fun. The minigame I played with my friends was the Zelda one, and it was BORING. But I'm still excited to see what the rest of the minigames will be like.
... Yeah, that's about all I have to say here. Oh, I got a Wii U shirt since I forced a friend to come with me