Good, but needs some balancing...
I think the concept behind it and the upgrade systema re all very well executed, and I like having the different weapons (different in the fact that they are unique and not just "cannon 1, cannon 2, cannon 3, etc...) I feel that there are some aspects that could be worked on to make this a truly exellent game:
1) The first thing that confused me was the fact that the red tanks are shooting at me even though they are spies on my side (as stated in the second red tank level). This makes it plain annoying to have an enemy I can't kill continuously shooting me.
2) The saw blades seem overpowered. The destructive side of me appreciates that they are basically a one-hit kill weapon, but the gameplay/challenge side of me wishes that it didn't take two simple hits to kill the end boss (he simply flew over me without me even moving and was dead in less than 5 seconds).
3) The physics of bouncing off of walls and enemies was a bit distracting. Since the main focus of the game is being the last tank remaining to kill the entire enemy force, bouncing off of everything didn't seem to flow with that concept. (Besides the fact that in real life a tank would simply crash through a building or run over a car).
Other than those things, I feel it was a solidly made game, and I also like how the upgrades and weapons are not infinite and have to be re-purchased.