Some of the worst 2D platforming and level design in any game period, but god damn the plot and music will keep you hooked until the end. Sucks that the guy who wrote the story for this game was "taken care of", whatever that means, by Nintendo's Lethal Injection Department.

We all remember the cool moments, because they're so cool that you forget how lame the rest of this game is:

Mario goes to hell before he dies? Remember how you had to do the boring Underwhere staircase two times, because you need to return a book? Remember the fucking Overthere apple level?
Slave labor and spooky spider woman? Remember the genius game design decision to make you run in a hamster wheel for actual 15 real life minutes with nothing interesting happening?
The final boss? Remember how he literally dies in 3 seconds so you had to pretend to struggle a bit so you could hear more of "The Ultimate Show"? (We all did this, don't lie).

Still good enough for baby me to finish this game 4 times, and I recommend people to play this game just because of how memorable the really good parts of this game are.