Ok, played last night, and loved it. We need to get a tigersx game night of this shit on Monday.
If you've played Borderlands, it's similar with the co-op drop/in out. One player, the host, has missions assigned, and you basically drop in and go along the same missions that he's playing. If he's too far ahead in the story from where you are, you don't get credit for completing the missions.
All in all, good clean zombie killing fun.