Wow! One of the best I've played so far! The only thing keeping me from a perfect score on this one is the difficulty. Goodness, even with the slower enemy setting on I barely scrapped through. Perhaps there is less of a mouse lag if it's not in the browser, maybe that's a factor, but it is extremely difficult pretty much immediately, which could push some more casual players away from the very unique experience.
Bonus: It is pretty neat to be able to play this using a Wacom tablet, or a similar tool. That is how I started out since I saw it was a Paint layout!