I like achievements. They bring little mini games and side quests to the game and make it more fun. Yes, some are ridiculous but some games do it well. And its' a nice way to call out people's bullshit. "LOL I beat CoD4 on realistic!!!" Quick check shows otherwise.
I like them too but not too fond how some of the MP ones are setup. Many times my on-line play has been ruined just because some other one is out say for a 50 caps in a CTF match. B t I did go back and try things that I might not have thought about before. I even played games more than once.. Like play COD4 on realistic just to see if I could (and I did check my gamer tag)