Tripware made a really good job with this game.
It can be quite boring the first time you play it, because it seems like you just camp and there is not that action other games like Left 4 Dead have.
Easy difficulties like Beginner or Normal are really easy, but recommended to learn the game.
When you start playing harder difficulties it becomes more enjoyable. Hard being quite relaxing because it's not too hard but it still got some exciting moments.
There are also classes (perks) which are a bit hard to level up. Not hard, you just need to play the game as that perk to level it up, but all perks are nothing special until you hit level 4 or higher.
But perks make the game more fun, forcing you to rely more on your teammates. Some people don't like to rely on other people, and that's the reason they don't like games like Killing Floor.
It's a decent game when you play with unknown people, but it becomes really fun and enjoyable if you get some friends to play with.

Very enjoyable the first time you play it, then it becomes repetitive because it's just a single campaign.
It's free, so this explains why it's not being updated by Valve too much.
Still, a good game to play from time to time.

One of the best games i've ever played.
It's not a serious game, don't buy it if you are too serious :P, you will have a lot of time to play serious, but the main objective is to have a good time with it.

If you played Team Fortress Classic buy it.
If you are getting bored of Counter-Strike or Day of Defeat, or you feel those games doesn't fit perfectly for you, you may find what you are looking for on Team Fortress 2.

Also, if you see Team Fortress 2 or The Orange Box on sale, you must buy it.