I went to an all-girls Catholic high school (I'm not remotely religious myself) and *loved* it. It was smallish (about 600 students), which was just the right size for me. True, there was a shortage of classes (they wouldn't let me do an independent Chem 2 study, and there was no AP Chem at the time), but the atmosphere was amazing -- very supportive, very congenial, very welcoming. I'd do it again in a heartbeat.
Ex-h went to the all-boy high school in town and also had a great experience. They had more class offerings than we did, so he didn't run out of classes (I think they had closer to 1000 students at the time). They didn't have all male teachers, and it didn't seem to make a difference. They did have a fair number of male teachers, though, which is nice for teen boys.
I'm definitely considering single-sex education for ds6 when he gets to high school -- there is one nearby that looks promising, but it's a long way off! For grade school, I think I prefer co-ed, but high school is the time where the, um, stuff hits the fan socially, and I think having a single-sex environment cuts down on a lot of the peer pressure to conform at school.
