It's been used for men for a couple hundred years, and on women for nearly as long. Right now, on girls it's kind of trendy, and on boys it's very outdated. YOu don't see a male Sidney under the age of sixty, it seems, but you see female Sidneys/Sydneys from birth right up to old age.
When I was little I liked a series of books about "ALl of a Kind Family." They were written by Sydney Taylor, a woman, who as near as I can figure was born right around the turn of the last century.
I consider it to be truly unisex, because at one time it was as common boys as it is now for girls.
However, having said all that... I don't like the name. The Sid sound is sly and sneaky to me. I picutre a male Sidney as being a smooth-talking, kind of oily, used-car-salesman/shyster lawyer guy. On a female I picture a snobby, "socialite" type woman who doesn't work, her whole occupation is buying things adn going to parties.
The Sid sound also puts me in mind of SIDS, and Sid Vicious. Sidney Biddle Barrows, the "Mayflower Madam" (she really is the type of Sidney I described, and got into trouble many years ago for running a call girl ring.)
There is Sidney Poitier, and some other Sidneys, and I know two Sydneys (both female) who I like, but I still don't like the name.