You'd think it would get repetitive and boring, but it doesn't. It's just awesome, and awesome, and awesome again.
Gameplay: Plays like Left 4 Dead but a downloadable version and against enemies that are slightly easier to kill but just as annoying. And that's a good thing.
Graphics: Looks great for a downloadable game with so much content and for a debut game from a company. It's also quite violent and there's no noticeable framerate issues from what I've played except from a long distance during Green Bridge, there's rain, helicopters, etc.
Sound: Hearing all the voice acting is great, the characters may not seem like they're coming to life but it still sounds great and the gunshots seem so real; if it weren't for the physics, they would feel real.
Lasting Appeal: It will last you a long time but make sure not to get burnt out, take at least a half hour break every two games to ensure the game remains interesting. There's also a Platinum trophy to obtain and of course, great co-op multiplayer which the game was made for.