In terms of design, it's good. I liked going through most of the demo, which is not bad at all. A few levels were less entertaining, like the castle level (the P-Switch backtracking was a bit boring to do) and the mountain secret level, which was pure sprite spamming. Either way, I had fun, which means that this hack isn't this bad in terms of design overall.
Nothing much can be said on aesthetics, where the only differences from the original SMW were about some custom palettes here and there. Some looked good, some others a little bit off (like the FG one when you get out of the water level). Not bad though, there weren't many flaws.
No issues found, but I'd like to mention that "Yellow Switch 1"'s secret exit leads you to an unedited SMW level. I know the author didn't want the secret exit of this level be in the demo, but you can still get it
if you have a cape and a Yoshi. You can fly and then dismount on Yoshi in mid-air in order to go where the red blocks wanted you to go. where the normal goal is.
Thus, I'm counting it as exit.
Overall, yeah, a decent small hack. Not lenghty, not perfect, but it's okay to give it a try.