I have around 12800G because I don't have so many games and I mainly just rent a game for 2 weeks, get some achievements in it. I think mine is about a third or a fourth of the possible gamescore but I've got no clue.
I'm hovering around 14,000, I believe. A little bit less. I don't really "go for" them unless I feel I have the actual ability to achieve them. But the whole "collect 100 feathers" is just a pointless timesink, not going to bother.
I have over 6000, but I only play a few games. As of now... L4D2 1390/1500 L4D 1020/1250 MW2 890/1000 Black Ops 725/1??? Lego Batman 675/1000 (just takes time to get the achievements)
And there are various others that I have some gamer score in, but just don't play a lot.
I've also got a PS3 with a bunch of trophies on, and I spend more time at the computer than the two consoles combined. Sorry, just felt like I had to make an excuse after hearing some peoples' scores.
Anyways, the only game I've ever 100%ed (DLC not included) is Dragon Age: Origins on PS3. I'm about 4 achievements away from 100% at Mass Effect 2 on Xbox360 too. Aside from in BioWare games (AWSUM), I usually don't bother to go achievement hunting.