Similar to other games.
This reminds me of quite a few defense games. It's good but I like the concept of waves a little better than a mind rush. The enemies become a little predictable and I could do a "spray and pray" technique of waving the mouse up and down, pressing the mouse and not worrying about anything hitting because I had a powerup.
I would have liked the idea of upgrades at my discretion. Engine repairs that keep the gun firing for longer periods of time, better fire power, and different enemies that take different amounts of damage. Of course, helpers may or may not make it but those would be welcome as well. Maybe have a civilian come out and shooting the planes with a laser?
Graphics wise, it's good. I just feel that the gameplay could be better.