Your game's nice in its current state, but it sometimes plays sluggishly when one ball's stuck in a corner: there's no longer any tension nor dynamism and it becomes a game of waiting till you land the proper hit. adding either a limit on the number of shots or a time limit (or damage given by the opponents) would give this game what it lacks. You could also add other game mechanisms (bouncing balls, exploding balls, double balls, etc) alongside the ennemy's features; it would make the gameplay more unique. A score / combo meter would also fix these issues (and be simpler to add).
Even though my comment was a bit harsh, I did give it a high score in enjoyement.