Beautiful and brilliant!
Now for the multiplayer. Yes, there is a host advantage and yes there is some unfair (and intentional) glitching going on in many battles. But besides this, the multiplayer is very good. The 4 vs. 4 Deathmatches of Warzone and Execution work very well, and the startegic points of Annex is also quite fun. The newer maps, eh, they're not really worth it. I actually purchased them today and I recommend you to wait until the new ones are out because they are not played very much (compared to the originals).
The co-op is a great experience whether you are playing it ith a buddy in split screen or playing on Live. The co-op gives you much more strategy (it's easier to flank with 2 humans) and also, some support if you are taken out (which is much needed on insane and maybe hardcore).
All-in-all, Gears of War is a great game, with some very minor flaws. If epic tweaks a few things and improves the story for the highly rumored sequel, 2009 or 2010 could see the release of the finest console shooter ever made.