I quote Valarian in everything he said. The only time I left a dungeon was when the healer started acting like a jerk and repeatedly insulted the tank - who was a friend of mine, but then again I would have done that even if I didn't know him. I have had some wonderful experiences even after being repeatedly wiped, an example would be almost a week ago during the Navel boss fight. It was hell, but we just repeatedly learned from trial and error without giving up. It felt good. Yesterday's Castrum Meridianum was horrible, because at half of it there was a mass abandon that tore the group - all because the healer wanted to rush, while some of us didn't even do it the first time. Damn, I didn't even understand WHAT I'm supposed to do because of that guy, at the end of the run! Of course, he was in the group that left. My rule of the thumb is: don't leave unless you want to strangle someone that wants to strangle you. If it's not reciprocate, we may still do it.
...I guess.