Best Co-Op game I've ever played with a marvellous community and its own subculture. It has been my absolute favourite game for a couple of years.
Stopped playing though when they introduced the "flappy boots" minigame. I know it's optional and there are mods to get rid of it, but somehow this has killed the entire game for me. Silly, but can't help the feeling.
In German I would say "die Luft ist raus" about this. Literally translated: "the air is out" and describes situations where something totally lost its appeal and is now just "meh", although it was (very) appealing before.
If you haven't played it before and like co-op shooters: give it a try. You probably won't regret it.