Every game for me. Achievements add an extra sense of purpose to games on top of all the other elements of the game. Particularly if I have several games to play, I don't want to miss out, so I use the achievements as a measure of how much "worth" I've gotten out of the game. Plus, I'm goal orientated most of the time and the vast majority of achievements I've gotten have been through playing the game the way I normally would have done anyway. Plus, some amusing or interesting achievements mean I might play a game longer than I would do after finishing it.

Dead Rising is easily the best example of this. So many fun achievements...

I still need to get Dead Rising. I was waiting to see how Chop Till You Drop turned out, but the verdict doesn't seem too good.

This