This mod is a gem. Hell i wasnt even a fan of the whole zombie genre when i tried this game out of curiosity five years ago but it quickly won me over thanks to its gameplay and atmosphere. Its immersive, challenging (teamwork is a MUST) and above all, fun.
An interesting aspect of the game is that you actually can play as a zombie after you died. No bot slaughterfest à la Left 4 Dead here. Its very enjoyable to fight a zombie horde composed of other players trying to overwhelm you so you can join their ranks. Zombos can also infect you which really adds to the tension since it creates a climate of suspicion among the survivors.
Most of the official maps are well designed and interesting. The character models are well made too and all have their own "personality" and dialogue lines which is pretty cool.
Last but not least, there is a really nice variety of weapons, including several melees. There is also a barricade system that can help you to hold off the undead. I should also mention the weapon and ammo management system. Guns and ammo slow you down so hoarding ammo is not an option. The weapon/ammo weight plus a fatigue bar adds to the realism and makes being a team player an absolute necessity.
If you are looking for a fun, teamwork oriented zombie survival game, give it a try. Chances are you wont be disappointed. And remember, its FREE. :)