It's popular for both, but not horribly (in the 300's for boys, 400's for girls).
I'd love it for a boy, and I'm sure it would go nicely with older names (aka,
Casey and
Eleanor,
Casey and
Edward,
Casey and
Rose,
Casey and
William)
However, I dislike it immensely for a girl, and I don't think it'll seem as nice with older names.