I was at a single sex school from 13 to 18. Some of our 6th form classes were mixed - mind you, one of my subjects was entirely at the boys' school, but was an all girls class, because no boys took the subject at A-level. I suspect these days, they wouldn't even run it as there were so few of us.
It was a small town, and we'd been at school with most of the boys up to age 13, and saw them outside of school at various activities (swimming club, youth club and so on.) There were also some mixed activities like French exchange trips, drama productions and so on.
There was bitchiness and cliquiness at times, but I don't think it was any worse than it would have been at a mixed school, with all the shifting alliances and so on (internet messageboards can be like school was in that way) - and at other times, it was very supportive.
I do think being at a single sex school helped with my confidence. There was no discussion about whether STEM subjects were not for girls or not - it was just a question of whether you were good at it or not. Mind you, I had a grandmother who had done maths at Cambridge and went on to teach maths and physics, and an aunt who taught chemistry, so I already knew lots of women doing that sort of subject.
We were told we were the academics and business women of tomorrow. And we were shielded from a lot of everyday sexism - when I was in the 6th form and we were at a planning meeting for an A-level French trip to France, I was so taken aback by the idea that the girls would be in different accommodation just because we were girls, I stood up and challenged it. I am not sure (but obviously can't know) that I would have had the confidence to do that if I hadn't been in a single-sex environment most of my teenage years. Also, if I'd been coed all that time, maybe I'd have just been so inured to everyday sexism, I wouldn't have been shocked or thought to question it.
I socialise well with men as an equal, but have less success romantically - however, I think this has far more to do with my mother telling me no one would ever love me and that if I ever went out with someone in the county, then she would know all about it, and being assaulted at 16, just as I was starting to gain some confidence, than because I was at a single sex school.
If I had a daughter, I would definitely consider sending her to a single-sex secondary school. It would depend on the school in question, though. There are good and bad single-sex schools, just as there are good and bad mixed schools, and while I think it was good because of the lack of everyday sexism and the expectations you did subjects because you were good at them, not because they were a subject for a particular sex, I think mostly it was a good school that just happened to be single-sex school, rather than because it was single-sex.