I've only played "The Hidden" and "Plan of Attack". The hidden is graphically pretty nice and the concept sounds awesome...however it just didn't pan out to be any fun to me. Its to hard to successfully kill anyone as the hidden and the best strategies for killing the hidden include standing around and waiting for him...which is boring. Plan of Attack just didn't grab me either...all it seemed to bring to the table was a built in strategy chooser, otherwise its a lot like CS:S. I don't know.
I'm waiting for one or two of the millions of zombie mods to come out with a beta, some of those have promising concepts.