I'm really enjoying this mod - it has a lot of similarities to Left 4 Dead, but it feels a little more geared toward realism - there's no HUD to see health, ammo, etc. You're very slow in this game, and weapons / ammo aren't incredibly plentiful. Also, if a teammate screws up a grenade throw, or you walk in front of someone aiming, you're likely to get killed! No More Room in Hell is pretty difficult, but a TON of fun to play. I'd really like to see this mod get popular!
This game has a sort of different take on zombies. In other games, you basically gun down any zombies that stand in your way, but in NMRiH, you try to run between zombies and around zombies to get away from them, since they move very slowly. This creates a lot of tension in trying to meet up with teammates or get to objectives, and there's a lot of potential for you to walk around a corner and shit your pants when you're face-to-face with a zombie. I've been enjoying this mod, and you should give it a try, too
Here's some of the fun my friends and I had playing this game!