Really enjoyed the implementation of the concept, especially loved the upgrades with penalties. I admit that wasn't clear to me at first, but it was AWESOME once I realized there were costs to my decisions.
I wasn't a huge fan of the controls, I found that the hardest part. Not sure if there are other control schemes here that would work, but the mouse aim up/down was fine, it was the strafing that was very unintuitive for me.
Overall great concept and build.