Hey ! First of all I didn't encounter any bugs during this demo.
Now my opinion, concerning the graphics, I find it pretty and original the colorful way as well as the animations of the enemies as well as that of the character we control. After the musical level, I love it! Maybe due to my love for Warioland whose style is very similar (A little normal since the game takes on a lot of the aspect of a Warioland). A small criticism I could make on this demo is the lack of difficulty. (SPOILER: I really like the way the level ends, with the "boss" who is just a simple enemy who is swept away with a simple punch).
So for me this demo is very good even if there is a lack of difficulty and it will be a favorite with Warioland fans. :) 