Sin City is not "an endless stream of misogyny and sadism" or "an orgy of violence against women." Violence against women is far outweighed by the number of acts of violence against men. And despite the movie's focus on the underworld (which would explain the limited number of career paths depicted), they are not all prostitutes: they are waitresses, law students, parole officers, and bomb throwing mercenaries. We never learn Wendy's occupation--she clearly knows that her sister is a hooker and is adept with a gun, but whether she is also a lady of the night isn't clear.
The women of Old Town are as liberated as a autonomous city-within-a-city of prostitutes can be. They pack heat and do a decent job of protecting themselves (although their self help is of limited use when they face those who enter Old Town with a species of diplomatic immunity, like Roark & Co. or cops out for women's blood). They are depicted as independent and relatively self sufficient, pragmatic and willing to make tough choices to preserve their realm, and not particularly bothered by violence. They are emphatically not just victims. At some point in the recent past, they elected to renounce that role and assert their independence from the mob, pimps, and even the city government. They accept help from male allies like Dwight and Marv, but they are not dependent on such assistance generally. (Marv may be a sexist with his "I don't hit girls" schtick, but he's on the side of the angels.)
But this male/female divide is really not what the power struggle in the movie is about, and focusing on the acts of violence against women blinds one to the larger context. The real fight is against those who wish to treat the inhabitants of Old Town as slaves or objects (as opposed to those who transact business with them or befriend them). The mob wants to enslave the prostitutes, Roark & Co. want to prey upon them, Jackie-Boy wants to use them to violently assert his masculinity and superiority (in the worst cop tradition, he claims that hookers can't say no). And we should note that not all inhabitants of Old Town are female prostitutes; there are male prostitutes there as well.
Sin City is the story of a successful war for independence. Most of the body count in the movie (and nearly all of the victims of that specific struggle) are male; the pile of bodies generated by Marv, Dwight, and Hartigan consists of men who seek to subjugate Old Town and their corrupt allies. A few women are killed, but in the end they remain in control of their own destinies, at least imperfectly. While it's no blueprint for a feminist separatist utopia, neither is it a misogynist fantasy. Any true misogynist would feel far too threatened by the women of Old Town to get off on Sin City.
UPDATE: Speaking of feminism, Andrea Dworkin is dead.