I remember when people called the Wii a gimmick. It has now sold over 70 million and inspired Sony to make PS3 Move, and probably why MS made (or rather bought the technology) Kinect. That is a pretty successful gimmick. And now again, they release a new controller and people call it a gimmick. (or some). It's far more then just another gimmick, and has more use then the Wiimotes because whereas some games don't work well with a Wiimote, all games can use the U-Controller. Even if it's just for your items and you use the buttons alone, it's still can be used.
I remember when the Wii was being called a gimmick as well. I recall dismissing those claims because a handful of games at the time made great use of the motion controls. Over time however, those fears have been confirmed. The Wii's motion controls (and the newly minted motion controls for the 360 and PS3) are a gimmick, and not a very good one at that. I can count on my fingers the number of worthwhile games that absolutely require the use of the motion controls to be a viable experience. On the other hand, I can think of dozens of games that I preferred to plug my Wavebird into instead (Smash Bros and Mario Kart being the big ones) or that outright suffered from shoehorned waggle crap.
Were there fun games? Absolutely. Did the fun of some of those games depend entirely upon well done motion controls? Yep. That doesn't mean that an entire system should have been built around it though. If the 360 and the PS3 have done anything correctly with their late entries into the motion-market, it's that they keep it as an optional toy and don't force feed it down their customers' throats at the expense of time-tested, working alternatives.
The Wii-U will fail to impress in the exact same manner. It'll be awesome for some games (just like GBA-to-NGC connectivity was, just like the Dreamcast's VMUs were), but it won't bring enough value to the majority of games to really justify its existence. And with the way those controllers look, I have a really hard time believing that they'll be useful for much else than a hand cramp.