Well balanced
Perhaps my favorite of the "Defend your" games I've seen. It offered a good initial learning curve, and to my suprise, maintained that learning curve later in the game.
To anyone having trouble, my experience has taught me the best strategy is to spend all your initial earnings on Mobile Flak Cannons ($6000 tank), and any extra earnings on vehicle upgrades. Starting with armor, then damage, then reload speed, firing speed, and thn veteran upgrade. By this time, the sky will be permanently black with flak.
You can then save your earnings and work on the remaining upgrades.
I felt for a while that the game then became too easy. However, slowly the levels began to get more difficult. Had I not read reviews to pass the time during this game, I would have given up earlier, but kyue376's review proved quite helpful.
Anyway, great game, and it offered a sense of balance that I rarely see at Newgrounds.