Like so many other bloggers (August, Julie, Matt, Josh) I put on non-pajama pants and emerged into the sunlight yesterday so I could see Sin City. Verdict? See it for the style. Aesthetically, the movie is like nothing I've ever watched before. And I'm shocked by how well it worked. Experiments that radical rarely run so smoothly on the first try, but Sin City's visual inventions made the film. The acting, too, was quite good, particularly from Clive Owen, who turned in a hell of a performance. So far as the dialogue goes, I disagree with August, the super-cliched 40's film noir was jarring at the outset, but settled into the movie perfectly. Unlike him, I found it so stilted as to be totally natural -- it was just part of this world, and in that context, I couldn't imagine the characters speaking differently.
That said, Sin City is enormously violent. More violent, probably, than any movie you've ever seen before. There's more torture, both physical and psychological, than you're apt to find anywhere outside of a Wolfowitz hearing on Saddam (oh, snap!). There's a lot of pain, a little redemption, and no happy endings. Some like that. I'm not one of them, but that's my quirk, not the movie's fault. Do trust me though on the film's essential, total darkness. Sin City is not a Marvel comic on the big screen, so don't go expecting a run-of-the-mill action flick with a more enthusiastic fan base. It's a descent into a netherworld packed with evil, corruption, and sadism, so expect the flick to have some weight.
All that said, see it. It's too inventive of a film to miss.
Update: Josh's review goes much farther than mine in condemning the violence, even professing concern for the souls of people who can enjoy so much blood and gore. I'd remind him that this was an outsized cinematic fantasy land that was overtly presented as such. Cars bounced, speech is cliched, bullets are absorbed, electric chairs are survived, people jump down from rooftops, etc. The violence, thus, can be enjoyed as something unreal, it's not quite the same as cheering a real life beating. I'm not saying I always did enjoy it, but I can see how others would have loved it without compromising their "souls". See also Matt Singer's much more interesting response.