I played this from start to finish in like, an hour's sitting. I was so disappointed when I realized i'd done everything. Seriously, the idea behind re-using the stages with new gimmicks from 64 and Sunshine is worked perfectly into 2D, especially with them being based off of previous levels in the 2D Mario series. Plus, the controls. It's a big step up from NSMB, which was already a pretty tight 2D Mario moveset. The only tiny nitpick I could have is that the walljump detection could be a bit faster, for like, instant jumps from wall to wall. Just me being pedantic, though. Also the visuals were great. Felt like an official GBA era game.
Art: Great! Nice and simple without looking like hot garbage, and I like the look of the characters too.
Music: Fantastic! Doesn't do too much to futz with the song but still sounds like it's not just a straight remake.
Gameplay: Awesome! Wall jump took some getting used too, and running didn't quite feel the same, but I was able to play it like I play any other 2D Mario.
A small audio glitch on first run of the game, but it never happened after that on time.
BUT, there is one problem...did you HAVE to make shift jump?