I really enjoy playing co-op zombie mode.Other than trying to join a competitive community, Co-op Zombie mode is the next best thing IMO. Like many others, being able to actually connect to a server is a frustrating experience. So why those of us who enjoy it wait for a patch, here's the developer command to play it in single player without later having nightmares. I was going to sit through level 101 and try to fool people but had enough after level 62.
I've only made it to level 17 with the random people I have managed to play with so far. In SP, I noticed that after level 22, it sometimes took 2 shots with the ray gun if you didn't hit them directly in the head. After level 30, there were only 2 or 3 weapons that could kill them with just a few rounds. By the time I got to level 60, It was impossible without devmode being on. I took the time to pay attention to all this because i have seen many reports of 4 man teams claiming to have achieved high levels. I might be swayed to hear about level 30 and under. Anything over and I would assume that glitching or cheating was involved. We really can't go by screen shots either, just look at mine:

Even though many do not care for Zombie mode, I enjoy it and thought I'd post the info so others who enjoy it could play around.
So, Start single player and bring down your console.
Type in:
/devmap nazi_zombie_prototypeSome of the commands you can play with are:
/god
/give all
/give ray_gun
/noclip I would bind those so you don't have to type them in each time you need ammo or turn noclip on/off.
Couple weapons in there to check out besides the ray gun. When you use the "give all" command, scroll until you see the colt. There are 3 colts. one of them fires grenades just like a launcher in CoD4. Keep scrolling and you will be able to act like a Zombie. From your view, your arms are straight out and you use your melee button to strike at the other Zombies. Works alright in the early levels. then there's the now standard "default weapon". Your hands are out but no actual weapon is displayed. Instead, you have a crosshair and a silenced round which isn't too bad I guess.
Using noclip to go outside the building or on top can confuse the zombies a bit. If they can't directly get to you, they can jump pretty high and they look silly doing it. They will chase after you outside though.
Now you can go burn an hour checking this out for yourselves. All this applies for the standard SP game aswell. Simply load a mission, bring down your console and type "map" to see the name of the level. For instance, "Makin" would be /devmap mak
Have fun