Very nice, but some areas may need improvement.
The graphics are nice, the music is great, and the gameplay is solid.
The only thing I see that happens is mid-way through the upgrades you become pretty much unstoppable. To resolve this consider a class of tank or mech that is larger, stronger, and harder to kill - about 5 minutes after the smaller mechs. One that takes a lot of effort to take down. Additionally the options for sound, quality, and music should be accessible mid-game in case the units start to slow the FPS.
Otherwise you have a very solid base to perhaps add additional levels, which build on what the player has already done. Such as if they kill all the units for that level and have upgraded the base to max health, a new level might be a good idea. One that has more men, more tanks, and more helicopters if that is not too much work.
I have to say this is a 5/5 flash game and you have done a great job so far!