Orcs vs The Machine! is a well-crafted defense game, and the upgrade system stood out as a highlight. On the criticism side — besides the gun speed others have mentioned — I found the repair cost pretty punishing in the early game. That said, with enough skill (or stubbornness), you can build up decent gun power.
Once the difficulty curve is smoothed out, this will be an excellent way to kill some downtime. Music and SFX deliver, and I appreciate how distinct the orc types are — readability matters in a game like this.
Now we just need a version FOR THE HORDE.