I played the game a few days ago, and I quite liked it. The levels based on other Nintendo games were good (like the Link to the Past, Link's Awakening, Wario Land 4 and Tetris ones), the bosses were interesting (like Metal Sonic and Kefka) and the game itself was mostly enjoyable to play. I actually recommended it on raocow's forum as a potential Let's Play, although it beats me whether he'll actually ever make a video series based on it.
That said, I did have a few pet peeves:
1. The boss design was... average in terms of execution. Okay, I loved the concepts, but you really used Pix way too much, and it felt like you sometimes had ideas you couldn't really back up with the required amount of ASM skill.
2. The levels were mostly fun, although I do feel the design kind of got a bit underwhelming as you got nearer to the final world and 'ending'. It's better level design than say, Brutal Mario (which had absolutely godawful level design outside of being an ASM showcase), but it could have been made quite a bit better still, since a few levels felt a bit empty and unpolished.
3. The cliffhanger part in the game was annoying to me. Not going to say what part for fear of spoilers, but yes. I was a bit ticked off when the story took that particular turn...