I'd say it was definitely worth a buy if you're into that kinda thing. Zed Time, for a start, is really bloody cool. It's basically multiplayer bullet time, achieved when anyone gets a good kill - usually a headshot. This makes for a great few seconds as everybody lines starts taking a little more time and heads explode all over the show.
Perks also level up. The Beserker perk, for example, which is what I use, levels up as I kill specimens with melee weapons - the axe, the knife and the chainsaw. It makes for a nice little feeling of progression on the side.
On the other hand, the difficulty is a little squiffy. Each difficulty level affects both the toughness of the monsters, and the number of monsters per player each wave. Which sounds alright but the toughness is scaled to the full 6 man team - on normal you can get to the last wave alright if you're a good shot and have places to run to, but when you get to the boss (known lovingly as the "Patriarch", the lovable mutated nerd with a chaingun-rocket launcher for an arm) You. Will. Die. Which kinda mucks it all up because on the other hand, easy is far too easy.
There's also a good community, too, with several good quality maps already been released and a lot more on the works, with some heavy scripting involved (there is for example a beta of a map that basically follows the Resident Evil movie, complete with fully functioning laser grid of doom).