It's a good game, and worth the $15 purchase. Similar to Defense Grid.
I like how you can switch between RTS to FPS and turn the cannon you are focused on into long range; really changes your strategy. The game is hard enough that you have to.
Then again, maybe I just suck.
Maybe you good folks at AT could buy it, and then teach me how to play it?
Cheers-
Rakewell