Nice, fast-paced, and intuitive.
The only gripe I have (for the short & casual type of game it is) is the lag/choppyness and how it is treated:
The image freezes as the frame rate drops, BUT the mobs & bullets are still calculated as moving, despite the fact that the avatar is considered immobile. That's automatically fatal, and unfairly so, as there is nothing you can do about it.
So almost perfect score, but it needs a patch where dropping frame rate also drops enemy speed.