A 10/10 With friends. A 3/10 With other people.
As stated above, it's really fun to play with your friends, because it's a teamwork game, they won't let you down easily and the communication is real important too.
If you die in a campaign playing with other people you will have to wait around 10 or more of boring minutes before you can even play. (Not to mention you also have to wait when you just joined a server and waiting spectator mode) (So it goes like: Die, wait 10 minutes, restart, repeat.)
Whatever mode you are playing, weapons and ammunition will always spawn around you at the begin point and scattered throughout the map, not to mention, there is around 12 kinds of ammunition. What every round goes is like the following: people rush and take all the stuff they can, even if they have the wrong ammunition and gun, they will never share. (Some people don't even know how to drop it or use them!) Good luck there with your kitchen knife and a gun with no magazine. And guess what, melee needs real good momentum, and not everyone has that. People would die, turn into fast-agile like zombies which are really hard to deal with and most likely infect you and ruin your round.
Besides, if you play campaign. You could pretty much rush to the end point without killing any zombies.(Dodge and luring) But people just wants to kill them and eventually die by them.
If you play survival/defending with other people. They won't be defending ♥♥♥♥, but run around and solo the zombies then die like a fool. (Zone's get overrun, gameover. Restart. Repeat.)
You can't complete any campaign (Unless you are a skilled player and carry all their asses, like me, which has almost completed all the achievements) and survival/defending (Which has a 0% of winning even if you carry them so damn hard)
Basically it's a huge time waster game and ♥♥♥♥ people with an IQ of 10 (30% of the people) AND greedy people (60% of the people) playing the game IF YOU play with public people that is.
So get your 3 or more friends on skype/teamspeak, join an empty server with a cap of 7 or less.
= Profit. (Everyone is happy.
Only worth with friends. Trust me. You'll be remembering this review if you just join a random server.