It's a solid Mario game, but is the same tired formula, and is extremely difficult.
The Bad: Boost Mode can feel broken with too many players, Miiverse feels forced and annoying, extremely difficult and unbalanced, doesn't advance the series at all, we've played this before
Honestly, for such a "new" series it sure has repeated a lot. I think the New part should have dropped off long ago. We are seeing the same formula from New Super Mario Bros./Galaxy and it hasn't changed, not one bit. There are just a few additions to accommodate the Wii U gamepad and those aren't even that impressive or necessary.
If you have played games from the series you know what you're in for. This time around the game's difficult is ramped up to 11 and the addition of four player co-op can either help or break the game. The biggest addition is the Boost Mode from the gamepad user. You can see everyone on screen and can tap to lay blocks that your buddies can jump on. You can defeat enemies for them, help with bosses, stop items from running away, and even help block obstacles. This is both a blessing and a curse. With just one or two people it's easy, but with three or more you can't keep track of everyone. If you lay successful blocks you get a rainbow star which makes you invincible and you can defeat anything. A lot of times you will place a block wrong and kill everyone because Wiimote players rely on the Boost guy a lot.
It's also mindless fun, but the game's difficulty will make everyone throw their controllers. I restarted levels 20+ times because I just could not get through them. Nintendo continues to offer cheats, and the worst one yet is the Luigi demo. You can literally hit a green block and watch Luigi complete the level for you so you can move on. What kind of crap is that? Is this what we're teaching the new generation of gamers Nintendo? That games will beat themselves? Half the time I didn't care because after losing 25 lives and rotating the gamepad to friends we just couldn't beat the level. There is a large variety and there's some fun power-ups like the ice flower, but most people will give up early on.
Another feature added is the Miiverse. It's honestly just very annoying. People can post what's on their mind if they lose or beat a level and it just feels like Nintendo's version of Twitter, but 5-10 year olds can post on it so you get incomprehensible words, strange drawings, and it just feels really annoying. Of course only the gamepad player can post and see other people's posts. You can also play as your Mii which is fun, but these aren't anything that would make someone buy a Wii U. Once again, Nintendo fails to sell their consoles with their own games.
The graphics really haven't changed much from the Wii, they aren't ground breaking like Mario 64 or Galaxy was. I really feel this series is now dead in the water gameplay wise. Mario just needs a serious reboot. It doesn't help that the game is overly difficult and all the new hardware features are forced and can actually make the game feel worse to play. Even the star coins are nearly impossible to get this time, and there's still no reward for getting them all. Going from level to level just made me feel dizzy with Deja vu.
As it stands, NSMB U isn't a console seller, and doesn't advance the series at all. The Wii U hardware features feel forced and some times break the game. It's also the most expensive Mario game since the 64 era. $60 for a game you have played many times before isn't a bargain. If you are already tired of the series, this won't change your mind at all.