I like to play games as is, without really giving much thought to whether or not I've unlocked an achievement. But I've got several friends who avidly collect these virtual 'medals' of sorts, and who swear that this brings out a lot of fun in the games since some of the achievements encourage a wholly different playing style than a person is used to. Does anyone else hunt achievements? Why do you do it? What's the hardest or the most memorable achievement in a game you've ever managed to unlock? Or are you chasing a particular achievement now?