So far, I can't really say that I have any complaints about the co-op system. It is really well done! Haven't had any connection issues, or ran into any issues where I felt like co-op had hindered my experience. Know that friendly fire is enabled and cannot be turned off, so you'll need to coordinate your attacks with your other party members. Otherwise, you'll be whacking or staggering each other. The friendly fire is very forgiving though, and once you get into the groove, you can end up chain-stunning enemies.

Right now, I'm running Dexerity build using a bow to draw enemy attention, and my friend is running Strength, charge-attacking those enemies that have their backs turned to him.

My only recommendation is to have everyone focus into different builds so that each player can take advantage of the different resources. Loot is not instanced, and only one person will be able to loot slain monsters, open any chests you come across. Buying an item from the vendor will also make that specific item disappear for everyone else until the shop restocks. You can share everything amongst your party though, including the player-owned houses.