It depends very much on your class and the different personalities within it.
I've got some single-sex guided reading groups:
- L4 target: mixed
- L5 target: 1 girls' and 1 boys' group
- L6 booster: all-boys group
- L6: mixed
They work quite well and especially my boys have made great progress. They tend to be quite competitive and we don't have the issue of them thinking that they should sit back and let the girls answer everything (they are very capable of holding their own in a mixed group by now, but weren't terribly confident to start with).
In English, I tend to mix, but they sit either boy/boy or girl/girl (although I have a focus group which consists of just boys). In Maths, I usually have single-sex tables (the only exception being my current L6 booster group, who I have mixed and teach separately at present). This has been particularly beneficial for my lower-attaining girls, because they aren't in ability groups. They feel safer comparing their answers and getting things wrong when working with their friends and without having boys at their table. It also means that they challenge themselves more frequently and access tasks, which are generally at a higher level.
For everything else, they choose where they sit.
I have to admit, though, I haven't generally got an issue with my boys "messing about" or my girls "getting catty". My boys are very well-behaved on the whole. They work hard, follow instructions first time, are keen to do well and have developed into very independent and organised learners. They are an absolute joy to teach. My girls are very supportive of each other, work together well, start to get to the point where they chat a little less and get the work done, develop their tasks further and are also very keen to do well. They tend to be a little more floaty and go off into weird directions at times, but certainly aren't out to be nasty to others.
For my class, single-sex groups work well. I've had them for quite a while, though, so know them well. For other classes, which I have had in the past, that wouldn't have been the case.