Repetitive and unvaried, but I'm yet to get bored playing Killing Floor.
If you're not yet familiar with the basic concept of Killing Floor then in brief summary it is an online multiplayer survival mode game in which players can choose to take different roles (or classes) and work together to take down waves of zombies in progression of difficulty. There isn't really much more to it than that but the satisfaction you gain from taking the heads of hordes of zombies doesn't wear off as quickly as you may expect.
Killing Floor is the sort of game I would play I would retire to at the end of the day when I can't be bothered to rack my brain. A bit of pure brutal fun, a test of reactions and some team co-operation.