A great game as long as you have some buddies to play with.
The graphics are pretty good for it being built on the Source engine, which is several years old. You can blow off body parts and heads explode in a delightful manner with headshots. The sound is great and when the music picks up, you know a zombie horde is about to attack or a tank is charging towards you.
The only reason this game doesn't get a 10 is because it could use some more weapons and levels, in my opinion. You can use a pistol, 2 pistols, an uzi, shotgun, m4, sniper rifle, or auto shotgun. You can also use pipe bombs or molotovs. There are health kits and pills, which give you a temporary boost to your health. There are 4 campaigns with 5 levels each. There are 51 achievements but you can earn most of them fairly quickly. They are fun to try and obtain, especially some of the harder ones.
The verus mode is where this game starts to pull away from the pack, so to speak. Yeah, the zombie thing is fun and you feel like your in a zombie movie or something... but when you get to become a zombie and barf on survivors, pull them with a tongue, tackle them from a rooftop, or throw them off a skyscraper as a hulking tank, it's a new experience.
If content gets added to this game (come on, Steam), it'll last for a long time. As it stands now, it's an extremely fun game that really focuses on teamwork. Without the teamwork, prepare to quickly die.