As far as I can tell, they copied nearly every tool and menu and their behavior one for one. I'm not an illustrator so I can't tell how well they copied brush behavior for painting, but as far as editing goes, this does almost everything. One noticeable omission that I could find is lack of frame by frame animation and gif export.

I pirated photoshop when I was 12 and kept using that for image editing my entire life. The clone feels completely familiar and does exactly what my muscle memory expects photoshop to do.