[meta] the appeal of genderswap

I've been thinking about this for awhile, because so much genderswap coming across my flist in SPN fandom, specifically Winsister. I really don't understand why, though in some ways, it makes more sense to me that it would happen in SPN, where there is a distinct lack of female characters. (Though there are some awesome ones, but that's for another discussion.) Then there was this post looking for femslash perspectives on genderswap.

Now I'm curious. What do you like about genderswap? What don't you like? Do you have an opinion on it?

I'm not being flip, I'm really curious about this. So far, I've just not seen the appeal, but I want to understand it.