I can't decide if it's worth $2.99. I read all you do is camp and fight waves. And there was several people on the steam forums whinging about the sale cause it brought in n00bs.
It is just camping and waves. 6 idiots vs many hundreds of zombies. A good game is VERY tense. :)
noobs do make the game more difficult. Health & enemy count scales with the number of players. If you get a couple of new guys joining a high difficulty server you'll probably wipe. That's okay though, surviving isn't really the point.
Killing Floor is an OK time waster but it has that annoying Counterstrike mechanic where you earn money in a round and you can buy better weapons with it, but the next round you're broke and without a gun again.
Also it has a much too large recoil for my taste.
But for 3 dollars feel free to try it.
Meh, I guess for 3 I can give it a shot.