Thought this was worth mentioning because the SDMB has a group that (usually) plays a couple rounds of this on the weekend and the more the merrier.
Obviously there is a subjective assessment on whether a game is fun or not but if you enjoy this style of game (FPS) then this one is not to be missed. It is loads of fun and mainly in multiplayer (and I am not a huge multiplayer fan in general).
Single player is ok but multiplayer is definitely where this game shines. Hard to describe playing a zombie and the fun of puking/pouncing/charging/riding/tounging(smoking) the survivors (you just gotta try it to understand). Great fun though!
Anyway, the game is normally $20 and going for $5 on a midweek madness sale on Steam.
At that price it is hard to go wrong.
If you do buy it and want to play with other Dopers be sure to join the Straight Dope Message Board group on Steam and/or get in touch with SenorBeef.