Very mechanically solid, as Mario tends to be. This time tho it was coupled with some interesting level design and pretty impressive aesthetic polish, a nice change of pace from the somewhat stale New Super Mario Bros series. The new powerups weren't anything to write home about but they did make for some pretty interesting platforming gimmicks, especially the Bubbles.

The last challenge levels were the most fun I've had with 2D platformers in a while, and made me wish the Bowser fight incorporated any of those mechanics. Like, any at all.
New Nintendo tends to fail at final boss battles for some reason

Now that I think of it this game is pretty lacking in bosses in general