Pretty good, I saw a few places for improvement
Really nice game, really fun gameplay and nice graphics.
I have a few suggested improvements though:
1. The EMP(Space bar) code is better set up as part of a set Interval event as opposed to an EnterFrame event so that the user can only activate one emp every couple of seconds.
2. A slick semi-transparent HUD could add alot to the gameplay
3. The game's upgrade tree should be a little clearer as to what you have and should probably be based around the idea of letting you get as many upgrades as you want, but having the price double every time.
4. A load/save function would be a nice touch
5. Maybe it would be cool to give the enemy helicoptors their own weaponary, as well as giving the ground soldiers more sophisticated behavior. Also, the AI didn't feel "aggressive" enough. I know that's a little vague, but its kind of hard to explain. When one plays a defense game, one wants to kinda get angry at the oncoming hoard. The hoard has to be very zealous in behavior, and constantly attacking!
Overall however, this is an excellent game. It looks like its been a month or more in the works, so its good you had the resolve to finish it. I noticed this is in Armorgames, so good luck :)