After ~ 30 hours with the IWBMS (before the update of yesterday), here's my feedback from a game with a friend on a private server hosted on my computer. I don't have any logs saved right now in case you ask for it...
The players (NOT THE HOST) have some issues with a "black wall" if the player is moving to fast (with a vehicule for example). This one can be the driver or any seats, the world can sometimes won't load in time. This "black wall" is a tilemap/chunk (don't know how you call this) is going to be empty, without any entities in it. After a moment, this "black wall" is going to fix himself (after loading something I guess ?)
The HOST can't see the location of the car updating if someone is driving(not the HOST) is in this one, even by getting out of the car. If the host try to get in the car, he's not going to update the car. If the other player leave the driver seat and the host start to drive, the car is going to be teleported for all the players to the host perspective.
If you have way to much zeds ingame and you restart your game (or just to the main menu), well... All the zeds are gone.
If you leave the siren of a car ON for a long time, zombies are going to spawns until you turn the siren OFF. However, if you don't do anything you can end with 4k of zombies next to the car. All the other zombies are going to be retarded. They don't care of you and you own car until you kill them. This exploit can let a player able to loot the entire town without any trouble, use the bug from above and turn off the siren.
If you have too many zeds around (from 500 zombies ?) the loot table is going to break, by that I mean I can get 4 axes in 1 minute. I don't really know why, but this glitch is only with the axe x)
All the other bugs/glitch I saw have been fixed :b
We don't use any mods and we play with a average ping of 60 ping both of us.
Ho and one last thing ! It's nothing but with the sport car, the entry points for the seats are not in front of the doors.