After putting it on hold for a week or so to play Smash, I picked up New Super Mario Bros U again.
I thought that, for the most part, the game was pretty easy. Then I got to the Superstar Road. I consider myself to be above average at platforming games, and these levels are still kicking my ass, though in a good way. I finished the first four levels, and went from 99 lives to about 75 in order to get all three star coins in each. It's pretty difficult, and the level designs are stellar (no pun intended). My favorite so far is the second one, where you have to hit a switch to turn coins into blocks, and continually run to the next switch to hit it before time runs out. Nintendo utilizes this mechanic to its fullest; in order to get all three star coins, you must break one set of blocks and still make it to the next switch in time, make precise jumps at the last possible moment in order to continue, and wait until the blocks revert to jump onto coins, which turn into blocks when time runs out. Nintendo implemented every possible way to use the game mechanic creatively, and it was a success. As much as I loved a good portion of what came before this point, the last few levels have been a good step above them.
This may be the best 2D Mario game I have played. At the very least, I consider it to be equal to Super Mario Bros 3 and Super Mario World.