I was in a long-standing, long-running, relatively stable group with two leaders, one each sex and 7-10 members. I was in it for 9-10 years, it helped me a great deal a few times but was mostly scary (but like therapy is scary :-) I didn't have any problems with the people, made a good friend and the therapists were awesome (I've dreamed about each of them).

I was also in a group in college, with an actual/"real" therapist but it was lame, during a summer session and we group members literally never spoke; sat there and no one spoke, time after time. Too, the other kids were truly "weird" like only college kids can be :-) one claimed to have mafia connections, a couple others shacked up together (generally against group rules to see each other outside group) and it was truly a miserable experience.

A couple years after college I was in another group, the leaders were psych grad students I think or new therapists, two of them, guy (whom I had been seeing for individual but he made me join this group instead) and woman and they had no clue. There was an older woman, maybe 15-20 years older than I and she was projected all over the place, I don't remember if it was at me or another woman I became friendly with, I think me. She was all but physically violent but I never spoke (so it wasn't about me doing anything to "cause" it); it appeared to be about her daughter but the leaders did nothing (to either stop her or work it through in any way) so either I or the other woman was constantly shouted at; it was scary and bizarre. Didn't get much out of that group, other than relief the few times she didn't show up that week :-)

Obviously, a lot depends on the skill and experience of the leaders, who's in the group, etc. There's a whole lot of chemistry goes into it. A little like here but in person and "scheduled".