Went into my local Best Buy and got to play with the WiiU Station. It appears the only playable demo on hand is Rayman Legends. All other software showcased was shown in either video form or (in the case of mature rated games) slideshows. I'll give my impressions below in breakdown fashion.
The System
Very unobtrusive face like the Wii and appears to be about the same thickness as one, but almost twice as long in length. Could not get a look at the back where the I/O ports were due to the kiosk's setup. All in all, exactly what I wanted and expected from the physical look of the console as a device that I will have very little interaction with outside switching the occasional disc (going all digital for 1st party and 3rd when the option arrives) and sitting in my entertainment center.
The Game Pad
Shockingly light and a lot wider in person than I expected. I have big hands and they conformed lovingly around the base and trigger buttons with my thumbs resting naturally over the analog sticks. The analog sticks themselves felt tight and better than those on my PS3 DualShock 3. The D-pad felt great, not mushy but not super clicky either, better than that on my OG 3DS for certain. The face buttons felt good, not mushy, and were bigger than those on my OG 3DS. The Lz & Rz triggers felt good and my index fingers rested on them very comfortably. The regular shoulder buttons felt good and large. I transitioned my index fingers from the triggers to the shoulder buttons to get a feel of how far my fingers had to move or how awkward it may be and I was surprised to see that it wasn't bad at all. For a comparison, imagine holding onto an N64 controller by both of the lateral prongs and putting your index fingers on the shoulder buttons. I remember as a kid it being a somewhat awkward thing to do in game using those shoulder buttons because of the wideness of the controller. For the WiiU Game Pad, despite its incredible width, the shoulder buttons felt fine to reach. The stylus is non-retractable and fits in a slot on top of the controller like the OG 3DS. There is a volume slider on the top as well as inputs on the bottom. There's a camera in the top center above the screen. There's a dedicated home and tv button next to the off button (properly recessed of course to prevent accidental shutdowns) near the bottom of the controller under the face buttons. It's a pretty slick controller and I can't say enough of how comfortable it felt in my hand. This has surpassed the Gamecube's controller for me in both comfort, weight and ergonomics, which is high praise in my book.
The Screen
The screen is a lot wider and larger than I expected which was a pleasant surprise. There was no discernible lag between the screen and the tv in the Rayman demo I played. Watching the game on the screen was noticeably lower in resolution than the tv (which of course is expected), but the game looked absolutely fantastic on the little screen, much better than what I expected. For comparison I use the OG 3DS top screen in 2D mode for 2D and sprite games like Mighty Switch Force and the Rayman Origins demo. The game looked way better than both on the Game Pad screen with the colors being super vibrant and the play experience smooth. I couldn't imagine the same game running on the 3DS, for example, and look like it did on the Game Pad. Touch was responsive and I used my finger exclusively (the flat of my index pad, not my finger nails). It felt more responsive than that of my OG 3DS, which was even more responsive than my DSi before that. The only downside I found was the finger prints and smudge marks on the screen. It didn't interfere with control at all with me, but it did obscure my vision a bit in the glare of the store lights. This certainly will not be the conditions with which I play at home, but I will definitely be getting a screen protector for it. The gyroscope worked fine and was very responsive, almost granularly so, but wasn't overly sensitive.
The Demos
The only playable demo was Rayman Legends. In the demo, you had a choice of jumping into 4 portraits with the onscreen character to see a demo movie, change the character you played, or play one of 2 stages. You could also choose the options from a sliding menu on the Game Pad. The demo had multiplayer, but as I was alone I only played as such. This game is absolutely stunning! I don't know what it was, the proximity to the large 40"+ HD television, the vibrant color palate, a use of different lighting techniques for the game, I have no idea. I just know that this game definitely looks better than Origins, which is saying a lot. For reference, I beat Origins on my custom gaming PC playing it in 3D using Nvidia 3Dvision at a rock solid 60 fps on a 27" 1920x1080p monitor sitting less than 2 ft away. So i was thoroughly impressed that the game looked even better than it's predecessor on my machine. It played well with the Game Pad and I got quickly into the game. The two playable stages hosted different playing options. On the first stage (a normal Rayman level) about a 1/4 of the way through, they force you to use Murphy and you play the remainder of that stage on the Game Pad screen. It was a lot of fun, more so than I expected. I had heard that a portion of the game was suppose to play as such and I can happily say that it was as fun as playing the platforming character themselves, just different. The other character simply went along when the path was clear, or a platform was available to hop onto. The challenge in the touch sections came from multitasking taking care of enemies and bringing platforms in reach of your partner as well as playing with the environment to direct them to secret lum stashes and hidden rooms. This last particularly excited me because finding all the secrets in a stage was a huge part of the fun for me in Origins and I had thought with the Murphy sections it would go away. I'm ecstatic to say that it hasn't and you just have to be even more vigilant to not pass them by as you have no direct control over your partner and can't go back as a result of it. As for the touch control, it as exact, responsive, and I never had to push or mash my finger to perform a duty. Simple pokes and swipes were all that was necessary and I never experienced lag in the least.
The other stage was a running one like that of the (very tough but satisfying) tooth treasure chase in Origins except platformed to the beat of a song. It was significantly easier than the chases in Origins (how long they stay that way I know not) but still challenging and a lot of fun. It was a rendition of Black Betty sung in Raymanese if you will, and the whole stage had a very 'How to Train Your Dragon' vibe to it (especially playing as the new girl viking character) which I dug immensely. After playing both stages and watching the game play video twice (once on the tv screen and again on the Game Pad), I am extremely excited to buy Legends when it releases day 1.
As for the other demo videos on the home screen, I watched NSMBU, Nintendo Land, Lego City, and Scribblenauts Unlimited. All looked good in HD (especially NSMBU, geez what a jump from the Wii).
Other Stuff
The Home and TV (not tvii) button did nothing and the actually home screen, OS, or Miiverse was not present nor apparently accessible. Again, the real attraction of this system is the controller and I can understand now why they presented it the way they did back at E3 2011. Nintendo products (particularly in the last 10 years or so) are very hard to convey in words and showcase and presents strongest from personal experience. The WiiU game pad is no different and with the right demo, I challenge the most hardened of naysayers to play with the controller and walk away unmoved by the potential it brings.
As a PC enthusiast, a portable gamer 2nd, and console player distant third, I have long since stopped being excited for anything on the consoles despite owning all but a 360 and being a diehard Nintendo fan since the NES days. But incredibly, Nintendo has done it again and I am once more hyped to 'new Nintendo console' levels of hypeness for their next machine. These next 4 weeks are gonna be interminable.