This is one of my favorite Super Mario games and it's my favorite WII U game. A reviewer on Youtube said that when you beat it, you won't want to play it again, but that's not true. I have played and beat it at least three times. One great thing is that it gives even more levels to play after you beat it.
Like Super Mario Bros. 2 for the NES, you can choose to play as Mario, Luigi, Princess Peach or Toad. If you beat the game, you can also play as an additional character, 'Rosalina'.
You must collect green stars to be able to progress in the game. Each level has three green stars, except for some boss levels which gives only one green star. Some green stars are easily obtained but obtaining some require you to risk your life. You must have at least 170 of these stars to be able to enter the final level.
On this game, you can get a cat suit which lets you climb walls. The cat suit is needed to obtain certain green stars. If you haven't played this game, you should. I think it's excellent. I like it much better than Super Mario Bros. U.