Magic_Of_Light: I just finished coop with a friend of mine, and there is 2 glaring flaws to it.
1- You can only progress the hosts quest line, and there is sometimes passive rewards like hp, damage, armors for completing quests that the coop person will not receive, so you will end up having to help them do their story quests on their world as well, even if you are in the same faction.
2- You cannot access your stash from your world, the resources and gathering you do on their world and put in the stash there will remain there.
Also unique weapon spawns if both players want them will have to be farmed on 2 maps, which is a little tedious.
Easily the best rpg iv played in years, but make sure you have a good coop partner who you trust not to rob your stash, as they will have access to it. I would highly suggest using the DEBUG so you can access former saves just in case its needed, like if they rob your stash.
I've been playing it for a while, local co-op. I confirm what has been said above. I'd add that the GUI is minimal and that even in split screen you have enough space to see your surroundings. The co-op expierience is cool if you trust your buddy as you watch each other's back, which is cool if you want to check your inventory or craft and the other one watches out for wandering monsters.
When one character is knocked down inside a dungeon, if the other one exits then the fallen one is magically teleported outside. It's a bit like cheating, but I've read that this will prevent odd stuff happening because one leaves the area and the other one doesn't.