Achieves for single player games can be fun and add extra life to a game. You can complete DSK on 360 without getting them all and if your fond of the game it gives you a chance to go see what you missed out on in the first place.
Achieves in general for multi-player games is the only time the fall flat on their face because of achievement whores and people teaming up which is like cheating other people.
If your a real gamer you just get on and play, but as I said above they can add extra value to a game in a good way when used right. I would of liked to of seen DSK have 99 achievements, so much to see and do it would of been fun collecting them all.