A very clever premise, and sophisticated level design. I love the little "warning" signs that serve as instructions. :D
As others mentioned, there is some inconsistency between different visual assets, e.g. mixing line art with pixel art. That said, the game looks quite good, and the color contrasts are adequate, so it's never hard to tell what is going on. The map is very detailed, too.
A nice variety of challenges and hazards throughout the level.
The problem for me was that I couldn't figure out the most efficient way to possess enemies. I could get it done one way or the other, eventually, but they would often take a bite out of me first. And when I got to the point where you have to possess 4 mice, one after another, I just didn't have enough health to survive the segment...