It's New Super Mario Bros, but with some added features.
The New Super Mario Bros. franchise evolved when it made the leap from the Nintendo DS to the Nintendo Wii. New Super Mario Bros. U once again takes the franchise and changes some aspects, but retains the basic formula. Cooperative play used the Wii U GamePad in an intelligent way while playing single player was equally enjoyable.

I started out by demoing the game using the GamePad controller. The video streamed seamlessly on both the GamePad and the TV screen, so I could easily switch between both video feeds. The controller felt nice in my hand, but I was forced to use the D-pad; there was no option to use the analog stick. I feel that placing my thumb on the analog stick is more natural, so stretching down to the D-pad feels a little out of place. It's nothing huge, but the game would be more comfortable to play using the Wii Remote.
The gameplay itself wasn't terribly challenging. The first level featured a flying squirrel suit that allowed me to float around the stage. This made traversing the course much easier, and in a level as simple as the one being shown, there was no real difficulty. I believe it is best compared to the Tanooki suit from Super Mario 3D Land, in the sense that it makes platforming challenges much easier.
In the second level, a baby Yoshi was introduced. This Yoshi could inflate itself up like a balloon and carry your character over gaps, and into the air to reach coins. It was fun to once again use a baby Yoshi, and this short sampling has left me looking forward to the other colors/abilities. I did ask whether baby Yoshi could turn into adult Yoshi after eating so many enemies, and was told that Nintendo hasn't said anything about it “yet."

The last level featured some different platforming challenges, including a great number of spinning stars. There certainly wasn't anything wrong with the level, but nothing stood out to me as brand new. I love Mario platformers, and this was no different. The platforming was fun to work through, the music matched the levels, and the game had enough new features to engage me.
I was also given the opportunity to try out a new mode made available through the use of the Wii U Gamepad, Boost mode. In Boost mode, I was able to create blocks on the field to assist the player. I helped Mario reach the coins that were just out of reach, and other points of difficult access. It's a really simple gameplay element, but one that is novel and a decent amount of fun. I feel that placing blocks could get old after awhile, but not knowing what else awaits later in the game, it is hard to determine how in-depth this type of gameplay could get.
New Super Mario Bros. U appears like it will have plenty of content, and what I was able to play was enjoyable. 2D-Sidescrolling Mario, is almost always a pleasant experience, and this entry in the series is no different. I am looking forward to seeing what else this game has to offer. It is currently scheduled to release this holiday season, so hopefully there will be plenty more to check out about New Super Mario Bros. U.