Pretty good game! I like the idea!
My main complaint is that the game doesn't feel as dynamic as the game it's based on. Everything moves at constant speed. Would be nice, if speed of Xs increased as game progresses, and maybe they could be a bit varied in speed in general, which would make for more interesting gameplay. Also it could help make situations with 2 Xs more fair, in my opinion.
Otherwise, I enjoyed my time playing this game!