I went to a mixed junior school, all-girls 11-16 and then a boys school which took girls for the 6th form.
It was a long time ago, and I didn't want to go to an all-girls school at 11 as I had been very happy chasing around with the boys in the playground, but I am so glad I did. I felt those years spent without boys in the classroom allowed me to focus on who I was and what I liked doing without any distractions. I had no sense at all of what girls were or weren't supposed to like or be.
When I went to 6th form I found that I had to spend a lot of time pretending not to be clever, as boys didn't like being beaten by a girl. And suddenly there was a massive focus on looks and being pretty, which just hadn't featured before. I was always a good swimmer and swam a lot at my girls' school - I stopped as soon as I got to the 6th form as the spectators' gallery at the pool was always packed with boys who were allowed to come along and watch.
Socially I was fine at both, and even when at an all girls school I did socialise with boys outside school. I now have good friends of both sexes, so don't feel I suffered socially.
I definitely feel that there is a huge benefit to girls in single sex education. Academically there have been lots of studies to back this up - and there was recently something in the papers about the higher level of achievement in maths and sciences in all girls school (sorry didn't follow links above and can't be bothered to google right now...)
Anyway, I had to make the (fortunate) choice for DD recently between a good mixed and a good single sex secondary school. I chose the all girls school because she's good at maths and is likely to be good at science; because I think (like me) she would be easily distracted by wanting to be cool and be in with the boys, because I think she will do more sport there, and be able to get a strong sense of who she is.
Of course, not everyone will have the same experience but I found it really positive and, although it's early days, she seems very happy too.
I didn't let her choose - although I did listen to her opinions - as I didn't believe she could possibly know what she was choosing. She could only base her opinion on her own experience (of mixed school) and so couldn't possibly understand what she might have to gain from a different environment. She was happy at her primary school, so naturally she would choose to replicate that.
In fact her own preference was entirely dictated by wanting to go to the same school as her best friend - although she did also like the school I chose, and understood why I wasn't choosing the best friend's school (which actually wasn't even on our short-list).