Awesome game!
First off, for those of you who are complaining about only getting to fire one turret at a time... it's called stategy. Obviously, you're never going to need 2 machine guns. But the right balance of secondary firepower is the key to this game, and what makes it infinitely more fun. A machine gun backed up by sniper/lightning cannons is the perfect way to take out those annoying little enemies that are an absolute bitch to trace and shoot down with the MG.
As for the game itself, phenomenal. My only complaints are that there is an EXTREMELY limited number of turrets (there's pretty much only one or two FTW strategies, depending on how good your aim is with certain guns. No real chance to customize and play "your way") and it's a little on the short side. Other than that, this game was fantastic!