This game is one of the most entertaining FPS I've played in quite some time.
A vast amount of weaponry, classes and upgrades.
Co-op game play (play as a squad against the AI zombie hoard, limiting the "pwned" trash talk that dominates most human player vs human player FPS).
Excellent large scale maps that are beautifully rendered.
Well populated and friendly online community (you won't have a problem finding people or servers to play on).
Solo mode to help level up and improve your skills.
The ability to share weapons and cash with other players.
Steam based - love it or hate it - old school gamers (those gamers that used to buy an actual CD/DVD copy of the game) may find Steam I little different at first. Takes time to install and set up an account. Also, those of you running a software firewall (ie Little Snitch) will also have trouble with Steam and denying outgoing connections until to "allow" certain permissions. The game plays in full screen mode and you can't shuffle between other apps and the game. You'll need to quit the game to adjust your software firewall prefs each time your outgoing permission in denied.
Joining a game in one of the later waves will leave you horribly undergunned.
When you die, you must wait until that wave is over before reentering the match. The earlier you die the longer you are just sitting there waiting.
Some maps are quite dark and difficult to navigate.
For those of you that are easily offended or have youngsters, the game can be a little graphic, gore-wise, and some in game language may be unsuitable to certain ears.
All in all, this game is well worth the $20, With a high replay value, great graphics, friendly active community, tonnes of weapons, perks, skills and classes it'll keep you coming back for more. Happy hunting :)