I have my NAT open (although the hidden multiplayer detailed stats says I'm behind a cone nat). I have the latest model of Xfinity "One" router, I set my Xbox One IP to static 10.0.0.32 and put that IP in the DMZ, just to be safe, I also forwarded all the recommended ports for Xbox One, and I also enabled all the port triggering, and made sure UPNP was setup (ALL this stuff was untouched before Xbox One, it just worked on my Xbox 360 -- I think the router recognized the 360 UPNP calls).
My friend has his NAT open as well.
We can chat on the Party chat together but we can't play together, once again, we both have our NAT reporting (OPEN).
We both were past Rhonda's garage. He invited me, but then after it says its joining, it almost immediately says the connection to host was lost and kicks me back to my game. I need to endure the loading screen for about 30 seconds and then I'm dropped back in my game -- losing any progress I may have accumulated since my last save (WOW--just wow, what a design problem)
When I try to invite my party mate to a game, it almost immediately says that the invite failed -- he doesn't even get pulled out of his game, its like my invite didn't go anywhere.
Based on everything I have experienced on Xbox One, the very worst of it has to do with the Network configuration setup and the party system. I think the proof that this launch was rushed is right here in the multiplayer networking set up.
Please let me know if you have any ideas on how I can get the coop working. As it stands, I can't really recommend an Xbox One to my friends.