It's basically a film version of 3 graphic novels from the early 90's and looks exactly as you'd expect. Batman and Spiderman have both successfully been adapted for the big screen, but Sin City hasn't been adapted at all. It's been totally enlarged.
Each scene looks like it's jumped right out of a comic, which makes sense, as the director used the original comics as the storyboard.
It was shot in black and white, with splashes of colour, so most of the blood is either black, white or yellow (...it makes sense when you see it), and there's enough violence to satisfy anyone's inner sadist. My favourite scene has to be where a big dude (Mickey Rourke) is driving down the road with his door open, dragging another guy along the ashphalt by his face. Here's the original comic. The film scene looks the same, but it's moving :)
The whole thing was shot in front of green screens, so every background was added in later. The black and white is stunning, and any scene with a moving car looks incredible.
Verdict: You're not going to get a better big screen version of a comic. It's just not possible.