My roommate and I already beat this on normal, we're working on it on the harder level now. So far, No mercys finale was insanely hard because you are in the open and tanks wreck your shit if you don't light them up with a molatov. We just hid in the closet and took turns meleeing, then emerging only once the tank came and dodged him while he burned. Either way, most finales require a pipe bomb at the end to distract the zombies so you can get away.
Also, the airport sucked, really badly.
My roommate pointed out that this game may be RIPE for downloadable content. He pointed out that new weapons, NPC zombie types or even campaigns/levels might be possible, since this game did take us only 4 hours to beat on normal.
Amazing gameplay. My girlfriend says it's 'stressful'. Overall, I really enjoy this game which is pretty impressive considering I don't play many video games these days.