This is typical of any team FPS for me - it can be so much fun, or so frustrating I'd rather just quit out and go try something else.
I nipped into a public versus last night and two of the guys on my team were totally clueless. Despite that, me and the other guy did ok whilst infected, but as survivors we got spanked every time. Very frustrating. By the time we were approaching the finale, I was fuming at the other two and I had to leave.
Playing campaign can be just as bad, when people are lagging behind, or surging forward (and I don't mean just a 50 yards, I mean 200 yards behind for no apparent reason), getting caught out and calling everyone else noobs....
When it gets to the point I start taking a game so seriously that I'm considering verbally abusing someone, then I have to quit out
Saying that, I got into a versus game last night when it was set to Expert - neither team was lasting long - it was hilarious - but I just couldn't quit out