I checked out UC3 co op mode last night and it is tremendous fun.
I played a custom private match so as to avoid lag issues and thoroughly enjoyed it. The co op adventure mode seems to be based on UC2 maps if I'm not mistaken, which is slightly dissapointing but still good fun. But the most fun was the hoarde type survival mode. The rounds rotate from generic Survival to a mode known as Gold Rush and a zone type survival mode where you have to defend a set small area and only kills inside it count.
And the real fun is that as anybody that has played the Uncharted series knows, you can throw peopple around as well as shoot them, something that plays out well in the survival games and is far more gratifying than the basic melee avilable in COD Zombies for example.