Sin City is a weird movie. It's essentially three separate short stories which occasionally brush up against each other. That being said, it's a pretty cool flick. I think somebody finally managed to do justice to the comic book (or graphic novel) genre on film. Cheesier adaptations like X-Men and Spiderman were good in their own way, but were definitely flashy, big budget action flicks using storylines and characters from comics.
Sin City, on the other hand, is a (in the words of director Robert Rodriguez) translation of comics to the medium of film. The over-the-top fisticuffs and screeching car chases had the feel of living comic book frames. I've never seen the original Sin City comics, but the film beautifully captures (what I assume is) the ambience of Frank Miller's world, such as the bright white blood, the overt physicality of the characters and, most obviously, filming in black and white.
Definitely worth checking out, but avoid it if you can't handle some pretty gruesome stylised violence.