Yesterday a buddy and myself spent 6 hours playing COOP in Arma2 in a LAN. Ping was 1ms, and yet there was a lot of lag and jerkiness in the game.
One of the main attractions of Arma2 from a multiplayer perspective is the possibility to play the campaign in coop. Boy was I disappointed.
Maybe I just didn't get it right, but you can't actually play the campaign continuously in COOP, ie the mission just ends and then you have to chose another mission from the list. Its impossible to know which is the correct sequence of the mission from the mission names in the list.
Add to that that the scripting errors seem to get worse than in SP and you have a very annoying MP package, in my opinion.
I love the game, but BIS needs to fix that before I'll recommend my friends to play Arma2 again in our next MP session.