I have. My husband (well, he is now, he wasn't then), screwed me over royally the first time I played that game.
For those not familiar with the game, it is played on a map of Europe before WWI. The whole premise is that you can't win the game all by yourself, you have to convince other people to help you through offering mutual assistance against the other players, but you have to prevent them from winning at the same time. Lots of backstabbing. Poker has nothing on Diplomacy. I don't recommend it for people who can't leave what happens in the game with the game.