I'm hooked on achievements. And like other people have already said, I find they add an enormous amount of value to games - I find myself trying all sorts of crap I wouldn't usually try, and I get more out of my games that way. Achievements motivate players.
Having said that, I'm not much of a replayer. If I don't have all the achievements by the time I've finished my first playthrough, I've very unlikely to replay the game. There are some exceptions (I've played both Gears of War titles several times over, for example), but for the most part, I simply play too many games to invest a lot of time in any one of them. Move on, start something new.