I haven't heard any good female growlers, but I'm still a noob at metal, so that doesn't matter. In imagining what a good female metal vocal would sound like, I imagine it being higher pitched, but with a sort of witchish melodicness. Less fuzzy them a growl, and more seductive. Folkish influences to make vocals more melodic. I really don't think women belong to the stereotypical death and balck vocal styles, although they can if they want. Generally, women are better at some things, and men are better at some things. OF course. I just conjectured this whole post, so feel free to prove me wrong.