Like a British version of Left 4 Dead mixed with Horde Mode. A great co-op/single player only experience.
It's also pretty awesome that the game changes enemies and adds a new map specific to Halloween and Christmas.
Presentation: There's no campaign or story behind it all and so you start out completely clueless as to what's going on. It took me a while to get used to controls, enemies, environments which I've still yet to get used to, and buying from the trader.
Gameplay: It's fairly the same as Counter Strike in the sense that you have to switch to a knife to melee and other individual items and weapons. There's a lot of weapons and it's been updated since I've first played it so it's possible that there's more to come.
Graphics: Uses Unreal Engine v3 and looks really good. While not amazing, it looks really good for a game for cheap.
Sound: The characters sound the same but with so much to say, it's great to hear them speak it all. The gunshots sound impressive for such a great game and with a soundtrack like this, this game is an amazing deal.
Lasting Appeal: There's classes to level up and Steam has achievements. Plus there's special content on Halloween and Christmas. You'll be at this game for a while.