I think it's fair to say that folks who have acquired the full 1000 gamerscore (Or "Platted" for my PS3 fam) in Final Fantasy XIII or people who acquired the "Little Rocket Man" achievement in Half Life 2 are slightly obsessive compulsive. If you have no clue what the "Little Rocket Man" achivement is, I'm probably not talking about you. Over the past few years, you could easily toss me into the OC bracket."
When I first started playing games on my 360, I didn't care much about gamerscore or achievements. My first game was Elder Scrolls IV: Oblivion. I would have logged 200+ hours playing that game with or without the cheevos. As time passed, I became more aware achievements and my obsessive compulsive tendencies began to kick in a bit. Soon, I was playing games I didn't even like (and sometimes hated) for 50+ hours just to squeeze out that last 25G Achievement for finding all 700 collectible items strewn about the in-game world.
What Can I say? I have a very strong goal-oriented personality. There is virtually nothing I do in my life that I don't see significant purpose in dedicating time to. Cheevos scratch that itch in my personality. Over time, I began to focus more on acquiring cheevos than enjoying the mechanics and atmosphere of the games. Cheevos became a huge factor in my game purchase decisions. If I felt a particular cheevo was out of reach or ridiculously tedious, I would avoid a purchase just because I didn't want to have a 10G or 25G blemish on my completion record. I felt a game was never truly finished until that final achievement was unlocked.
One of the catalysts that lead to my obsession with cheevos was the subjective 'quality' of the games I was playing. I began to focus on achievements because I enjoyed obtaining them more than the gameplay surrounding them. I'd say cheevos are largely responsible for me being able to finish games like Venetica, Sacred 2 and Fallout: New Vegas. This issue doesn't apply to every game. Oblivion, Tales of Vesperia and the Mass Effect series are games I could play and easily enjoy without cheevos.
And achievements are a different animal for me. I don't care about my gamerscore or being in competition with other people. Achievements are all about the joy I find in completing projects. That's how I began to look at my games--like projects.
Recently, I've noticed a change in my behavior. When I play games I find boring or unenjoyable, my cheevo obsession does not keep me playing past my disatisfacton. Most recently, I experienced this with LA Noire. The achievements in LA Noire are fairly easy to obtain. However, I've found the game so profoundly unenjoyable that I just gave up and said "to hell with a the perfect 1000G."
Maybe it's my busy lifestyle that doesn't afford me the time to care as much about cheevos. I think it has more to do with the fact that I've come to grips with the truth. Completing all the achievements in a video game is not really accomplishing anything. Especially when to do so requires herculean efforts and investing hours in the double digits. It's just not that fun any longer. I've got more important and enjoyable "projects" that fill my time now.
That's not to say that I no longer enjoy getting achievements. I still try to fully complete most games I play. I'm just being a bit wiser about it. If I don't enjoy a game enough to "1G it" (getting the full 1000G gamerscore), I won't force myself to do it anymore.
Still, Star Ocean: The Last Hope stares at me every time I open the sock drawer where I keep my games. 1G-ing it requires a 4-hour bonus dungeon run with no save points. Failure means the complete loss of 2-4 hours of my life. I've been reluctant to go for it because if I do fail--that's 4 hours I could have spent playing Kinect bowling with my son.