Pretty good, but there's a game-breaking bug with the pause function. If you pause the game, then click outside of the window, it gets stuck in pause mode. Pressing Enter to unpause advances the game by one frame, but it also ticks down the timer by one. So after about 200 or so presses, regardless of where you are, you're dead. And after that point, the game gets stuck in a loop and won't go back to where it was. I'd made it to the third screen of level 1 before this bug bit me.
I applaud the Nintendo Hard idea, but let's make the game Nintendo Hard because of challenge, not bugs, eh? ;)