DC member Skwire and I have been playing through the co-op campaign sporadically over the last couple of months. It's two-player only online co-op playing through the entire first-person game.
As someone who doesn't play multiplayer online games much at all, I have to say I have never had a better time playing an online game. If you love sniper games, this is an amazing one, and the 2-player online co-op is a joy to play with a friend. Slow paced, methodical, atmospheric, tense, the whole kit.
ps. Sniper Elite v3 comes out this month.