The city is, indeed, rather sinful.
So what is this movie like? Well, in a nutshell, it's superb. I went in expecting, at best, a mediocre experience. This is both because it is a comic-to-movie conversion, which typically isn't a fruitful endeavour, and because it's more or less within the noir genre of film. Don't get me wrong, film noir is pretty neat when done properly, but it is distressingly difficult to do properly. Nevertheless, Sin City manages to both accomplish a decent comic-to-film transition, and faithfully reproduce film noir, all at once.
Moreover, the writing is simply excellent. The character narration ranges back and forth from dramatic, to melodramatic, to comic, to absurd, to reasonable, often within the same paragraph. This is definitely a thinking person's movie, including such convoluted lines as, "I'm about as good at calming scared a 19 year-old as a a palsy vistim is at doing brain surgery with a pipe wrench." That's a far cry from typical action movie fare like Arnold's "You're luggage," or Casper's "Kill 'em all!" This is a movie written by, and for, people who are in love with language.
Beyond the writing, the movie has a certain style that at first jars, but then satisfies. The vast majority of the film is shot in a smooth-grained black and white, with color added to particular features, like eyes, dresses, or hair, to emphasize characters. Excellent use is also made of reverse colors, and stark white-on-black animation. Added to all this is excellent acting, and action sequences that even self-indulgent fuckjob Quentin Tarantino, a guest director on the film, couldn't screw up. In short: this is a movie that does everything right, so long as you don't object to nudity, violence, lots of dialogue, more violence, black and white photography, unrealistic feats of strength, and copious amounts of violence.
Oh, and it's a little violent. And bloody. And gruesome.
So, if you think you have the stomach for it, I strongly recommend you go see Sin City, especially now that it is at second run theatres and can be seen on the big screen for a reasonable price. It might turn your stomach at times, if you're faint of heart, but it's an experience that nobody should miss.
Just don't blame me if you have screaming nightmares featuring Elijah Wood. That little hobbit is one freaky bastard.