I typically try to get half of the achievements and half of the possible gamerscore for XBLA titles and 75% for retail games.
Some games are nearly impossible to do that with unless you spend a ton of time looking for things that do nothing but give you an achievement so it just doesn't seem worth it anymore after the "get the orbs" challenges in Prototype, Assassin's Creed (flags, Templars) and Crackdown (especially Crackdown, though).
I broke 50 as well a few months ago and I can't say I'm not proud of it. I use it as a guide when I'm bored and want to check out who I'm playing against in like Halo or Call of Duty to see how familiar they are with the system as a guy with <1000 gs might not be as knowledgeable of a player as someone with 20k, you know?
I also like being able to compare achievements. I'm OCD all the way too so making sure my GS ends in a 5 or 0 is a priority. I had been going crazy because UMK3 ruined it for... years, actually but I finally found a friend with the game and had her help me get a couple achievements so I could round it off again.