I am always dying from falling and newer from health lost. Its hard to grasp movement. I wish at least the map was bigger, than I would have more space to evade enemies and concentrate on movement.
Also, because enemies are moving diagonally (directly after me), I feel entitled to have ability to shoot anywhere where I point (as opposed to shooting on cardinal directions).
Enemy spawns are too unpredictable. In other words, I didn't like enemy appearing right before me in a middle of a map. I would suggest spawning them (since they are flying) on the outer bounds of the map.
Overall, it looks like a good start. IMHO it has to improve a little bit to create a sense of flow.