Biggest Problem I Had
The biggest problem I had was knowing far how to avoid bullets. When you have upgrades and those little people are flying around you, it is very difficult to see just your own helicopter because the "helpers" are the same color as you and move the same way with the spinning. Judging by the way I played the game, it appears as though these "helpers" are invincible, so there is no need to have them avoid bullets, which makes it very hard to just move the helicopter alone away from bullets, because it is difficult to see just the helicopter. I would suggest possibly changing the color of the "helpers" to fix this problem.
Another problem I encountered, was that I was unsure if the spinning blades of the helicopter were considered part of the helicopter, and if they were hit by bullets, you would die. There would be some times during the game where bullets would pass through the blades, and others where the helicopter would blow up if a bullet touched the blades.
I know some reviews suggest changing the colors of the background and the bullets, but I think it gives the game its personality. It looks nice.