Horde mode was a stroke of genius on someone's part. Absolutely brilliant crack. However I find it can take a while to actually get a match going and the constant read errors on my disc are annoying as hell.
Liked the single player mode as well but the gaps in the story annoyed me.
I love a good story in a game and it didn't come out well in GoW2.

In single player scripting and team AI seem to be flawed to the point of being idiotic. On a number of occasions the teammates would run ahead like 2-3 meters and disappear behind a bend. when i proceed to follow them, some big nasty gets spawned and i have to fight it on my own, while my team is nowhere in sight. once i defeat the big nasty, i finally go around the bend and there they are, my awesome team.... sitting on their asses... where were they while i had crap kicked out of me? also, sometimes im advancing on the enemy and my team is sitting behind cover. after proceeding through a corridor or two and clearing out a gaggle of bad guys, suddenly my team magically appears right behind me. what kind of crap is that?!! also, the game suffers from something i used to see in older games where no matter how big is the group you are with, the enemy always starts to pick on you. it's like it knows that if they kill you there's a good chance youll have to restart the game :\
the other thing that bugged me is the limited colour palette. seems like the entire game is coloured with shades of gray or brown. say what you will, but it gets very tiresome on the eyes.

While I enjoyed the game, it's definitely not game of the year in my opinion. The current matchmaking setup is pretty weak, and the story was pretty pathetic. They randomly add new characters just to kill them off quickly and act as though I should care, and the bit about Dom's wife was really poorly done. While the gameplay is as good as Gears has always been, it hasn't really been improved either. I'm not a big fan of being 100% stuck to the ground, and having invisible walls everywhere. That being said, Horde mode is probably the biggest addition to the game and makes it worth playing if you can get a 5 player team together.