Very good!
Uhh...to all the people who say it isn't hard, try playing medium or hard! Then it gets hard! I think you need to up the base damage a bit. It takes too long to kill the planes with the amount of times that you hit them. In real life, it would only take a maximum of two shots. That being said, they are extremely inaccurate in real life so I would up the starting damage, but lower the starting accuracy. Without any upgrades my accuracy was fairly good. I didn't upgrade it until around level 12.
Otherwise, a great game!