I played through the first world, and I like it so far. The first Toad house level was hilarious.
I have one nitpick (and somebody's probably already mentioned this), which is that the fish sprites in "Burglar John" have no black outlines; it looks off. Also, the animations on some of them are glitchy, but I'm assuming that's SMBX's fault.
I might post an actual review once I finish it.
EDIT: reghrhre why do the Cobrats hurt you when you land on them ;__;
EDIT 2: During the first Waluigi fight, you can get trapped and forced to take damage (this part scrolls left): http://i.imgur.com/uqMn5GP.png
Yes I cheated but only because I didn't feel like fighting the boss again to take a screenshot