I think it depends on the child and the family situation too. My girls will not be going to a single sex school - unless of course for some reason it is recommended for them further down the track. Not having brothers I think means that i would like them to have male contact with their peer group in a normal social situations, which I don't think single sex schools provide.

I went to a co-ed school for that very reason and still have many male friends that I made during that time. I also think that my relationships with boys was much more balanced than most of the girls from the single sex school - they tended to get a bit silly around the boys. But that's just my experience and my best friend went to an all girls school and she turned out fine (although she played judo all her life so was surrounded by many brother-type figures and male friends) - so it depends on what is going on outside the school as well.