I never got into GoW on Live. It just doesn't have as much depth as, say, Halo's multiplayer. I only ever played with friends, maybe a couple of ranked matches. Tiedgen (I think you probably know him, Stas) is extremely good at it. He's in the top 400 worldwide for the gametype Annex, which isn't that special, but he's still awesome at it. He has all but two achievements, one being the 10,000 kill achievement and the other being "Win 20 rounds of multiplayer matches in Annex by fewer than 5 points" which he tells me is hard too.
I just calculated my average gamer score, and it's around 200. That's shocking. I need to pleh moar 360.