Concept's quite good. I loved flipping between the two sides to hit enemies easier. Art is in a very good 3-bit style, and the music and sound effects were used quite well.
Two issues, though. First is that it doesn't reset when you die, so you have to reload the page. That's really, really annoying and was enough to put me off of playing too much.
Secondly the controls were a bit weird. Using W to flip felt really strange, and I never fully got the hang of it, mostly due to the no resets thing. Swapping W and Spacebar would feel more natural, I think.
Overall, the game is still pretty good, it just needs some polish!