The worst part about zombie mod was that...it sort of sucked! Obviously, the idea is there (there are tons of zombie mods in development, and valve has a similar game, etc) but I really felt like it just wasn't enough. Its just a server side mod so you can't really expect much...but the zombies should really be AI controlled or something because it sort of sucks being one. It could be made pretty much completely impossible to win as a zombie on some maps. And there's usually only a handful of really good hiding spots as humans, which end usually end up crammed full with way more people then they can really contain. And of course, that just increases the chances that either a griefer or a noob is going to blast the barricade.
People are douches in that mod too. I hate when I shoot a soda machine over to that sideways truck and then other people shoot it away just as some desperate guy tries to get up. (If he has zombies all over him, thats one thing...but most people seem to say "I'm up...fuck everyone else!")
But I do agree with ZzZGuy...I remember that feeling of not wanting to look back as you hear the trademark screeching noise.