Oh. My. God.
I only toyed with the single player (and managed to hit both saferooms on the first run throughs on normal), but I can see this being a game to play over and over and over again with buddies. It's just go such a tension, a frenetic pace to it that it just screams replay. There are some VERY small things I'd add that I personally think would make it perfect (character customization, 'fireman carry/shoulder drag' for wounded teammates, and a bit more interaction with the world in the way of things exploding when you shoot them), but as I don't know what kind of limitations they hit in design, I have to argue this is about as close to genre-perfect as one can manage.
And the idea of controlling the hordes (if that is indeed how it works) is just too good to pass up. Even the demo, controlled by the AI director, managed to keep us on edge- and by us, I mean me and the 3 bots with me... surprisingly enough, they're effective at watching each others backs, and even managed to rescue me when I got in deep a couple of times).