This came up in conversation yesterday. Me and my brother went to an all boys secondary school to complete our education and both of us hated it. However, our whole school experience was skewed by our cult upringing - especially with the 'end being just around the corner'. So perhaps I'm not in a good position to comment objectively.
Our - never dubbed - cousins on the other hand went to an all girls school and as much as they loved the boys they claim that their girls only schooling was a positive thing for them.
Surely it's just an unnatural situation though?