I was mixed until I went to secondary school and then it was single sex - and I think it was a good thing. We were definitely brought up with the idea that we were the business women of tomorrow, and there was no nonsense about boys' subjects and girls' subjects. We did maths and physics and metalwork and woodwork, as well as needlework and cookery. (At least up till GCSE options.)
We saw boys out of school at swimming club and so on, not to mention on the school bus, for those of us from out of town. There were shared drama thing, and we shared a French exchange, so it wasn't total isolation. Also, half my 6th form classes were over at the boys school. I was quite taken aback when one 6th form trip was being planned, and we were told the girls wouldn't be getting individual tents like the boys would. I don't remember having been aware before of being treated differently just because I was a girl, and it never occurred to me not to challenge it. I suspect if I'd been in mixed education all the way through, I would possibly not have noticed so much, and have been less likely to challenge it.
There was some bitchiness, but I am not sure it was any more than there would be among girls at a mixed school - possibly even less. I think that's just a feature of adolescent girls being together. I'm still friends with some girls I was at school with, a quarter of a century on, so it can't have been all bad.
One way or another, I suspect being educated at an all-girls school has something to do with me now working in a very male-dominated area of IT. My current employer is really pushing for diversity at the moment, and I'm involved in work (on top of my day job) to promote STEM careers, particularly (but not only) to girls.
All that doesn't mean single sex education is always good - like any other school - all boys, all girls, mixed, grammar, comprehensive, academy, private, whatever - some will be better than others, and I don't think it will ever be down to a single factor. But had I daughters (which I don't), and had I choice (we didn't - there was one secondary choice for boys, one for girls, unless you could afford to commute and/or go private), all else being equal, I would prefer to send them to an all-girls school than mixed.