I just finished playing through the demo!
What I liked (subjective opinion, take with grain of salt):Overall, this is some high-quality stuff. The sprites and backgrounds are polished, the music is fun to listen to, and the controls feel really polished. After taking a look through the last 10 years of this thread, I can see that a lot of work from the developer and feedback from players went into this.
There are some creative levels in there. For example, there's one where you control one of the main characters over a level that is completely covered in spikes. This character can walk on them without taking damage because she's wearing thick shoes. There's also a level that kind of plays like the Wheelie Kirby stage from super star ultra. That's the only stage I played multiple times just because it was fun; and not because I got several game overs.
What I didn't like (also subjective opinion, definitely take with grain of salt):I don't have a lot to say here. There is one thing, but it didn't bother me that much - some bits of the story kind of seem to come out of nowhere. For example, there's a level where there's a giant buried person - and that was just a background element! There's also a level where this lady in a giant robot mech is chasing you through ancient ruins. That kind of came out of nowhere too. But random stuff like that is normal in platforming games, so I don't really mind it too much.
Glitches (objective fact - the part you will really care about):There's a glitch where if you have the shield and boomerang abilities at the same time and you get bitten by one of the mushroom enemies, your sprite disappears and you can't move. You can swing the sword, but that's it. It looks like this glitch has already been mentioned by someone else in this thread, and it's probably been fixed already in an unreleased build.
There's another glitch that I haven't seen mentioned yet. In the second-to-last level in the demo, you get chased by what appears to be a giant robotic version of Jack. It's kind of similar to the level with the Mech Lady I mentioned before. Both can break blocks, and touching either is a one-hit kill. Except when Robo-Jack touches Jack, he doesn't just die. In every frame of the Jack's death animation, Robo-Jack hits Jack again, and it starts to lag (maybe a link to this screenshot will help explain what's happening?).
https://imgur.com/a/YRyhBXZThe weird thing is each hit counts as a life lost, which means once you're kicked out of the level, you have -26 lives (or something negative) left.
So it's really 2 glitches: First, Robo-Jack kills Jack more than once at a time. I've tested this twice, and it's happened both times. Second, you don't get a game over if your life count is negative - kind of a fun side effect, but it takes away some of the challenge. After you have negative lives, you can't get a game over because your life count will never hit zero. Of course, if you reset the game your life count goes back to normal.