An original concept, and reasonably well executed. The intro video was a nice touch. The visuals, while simple, contributed to the overall atmosphere, which was also helped by the excellent musical score. The theme from the final battle was particularly impressive, as was the gameplay of that battle itself. On the whole, I enjoyed this game.
However, it did have several issues.
The difficulty curve was decidedly on the unforgiving side. There were certain upgrade paths which, if taken, would effectively render the game unwinnable and require a restart. Most notable was the light beam. The loss of this on wave ten seems cruelly unfair to players who had spent significant resources upgrading it (up through wave nine, no less!) -- especially so given that the stealth bombers are still present and, after this, can only be detected by firing indiscriminately into the air at the risk of hitting a medical helicopter, or else waiting for them to drop a bomb on you. This would be mitigated somewhat if the concept of selling back an upgrade applied to all upgrades (which would also prevent players from having to restart); however, as it is, one finds oneself stuck with wasted upgrade credits and will likely have to eschew the light upgrades altogether on subsequent playthroughs. This is a pity, as it seems to invalidate one of the central game mechanics.
The game also provides no real indication of the effect an upgrade will have. Even after finishing the game twice, I still find myself somewhat perplexed as to the whether the shield disappears after a certain amount of time, a certain amount of damage absorbed, or whichever comes first. In the case of the main weapon, each upgrade changes its mechanics significantly and requires relearning. It also becomes more difficult to control after each upgrade, which is most noticeable in hard mode. (Perhaps some kind of target-locking mechanism would be appropriate? Say, as an additional upgrade offering?)
The controls were somewhat buggy, and frequently I would find myself unable to move the tank for a few seconds. Also, the game did not pause in the event of losing focus, which caused me to die a few times due to interruptions and accidental mis-clicks.
One last complaint is that the medical helicopter is difficult to recognize when it first starts coming on screen -- especially after the light beam is gone. As this game seems to encourage a reflex of starting to shoot something the moment it becomes visible, I found myself blowing it out of the air several times simply because I did not know what I was shooting at.
The combination of all of these flaws causes a rather significant frustration factor as the game goes on. This is most noticeable in the case of wave 11 on hard mode, which I would say is not so much challenging as unenjoyable due to the inability to see where anything is and the difficulty in hitting the targets even after you do know where they are.
As a result, even though in truth I like the game, I can't give it more than 1.5 stars. It really does need some work.