I checked it out yesterday. My impression:
Based from impressions online i thought i'd see more than just a fan-made level pack. Stupid me.
I played up up to level 6 in the first world, i think. Bridge level was the first one i encountered a genuinely new idea in. It had alternating paths for flying platforms. You need to set a specific combination of switches to get platform in position for you get second star which is inaccessible by any other way. I think, the way first star is positioned, collecting all stars on that bridge will require several playthroughs.
Though there
were some intriguing Super Mario World-type switches in earlier levels, so maybe there will be some delayed payoff... And overall a lot of SMW inspired stuff, like Yoshi's house and entire intro...
I appreciate fans managing to finish anything and actually releasing stuff in working state but i wish i liked this more. Will return to it later today, to check if i missed some awesomeness.