I am an achievement whore and a completionist. I'm a little OCD about it though. I hate online achievements. Absolutely hate them...but if I play a game on my main profile, I HAVE to get all of the achievements for it. It will bug me endlessly if I don't. I can't stand to have an incomplete game tied to my account. Therefore, if I want to play an older game and it has online achievements, since the community is most likely inactive I will log out of my main profile and log onto an "Offline Profile" so that the game won't appear under my Live account.
And I will actively avoid games that have glitched achievements that cannot be unlocked. Ghostbusters, I'm looking at you. I will also avoid games that have ridiculously time consuming achievements...Bomberman Act Zero, for example - I refuse to play it because of the achievement where you have to blow up one million soft blocks. I heard it would take months upon months, maybe over a year of grinding to accomplish that. Chromehounds is another that I won't play because of the amount of time it takes to complete. It's estimated over 900 hours to earn all of the achievements.