One of the main reaons I watch films is for the visual onslaught. Shitty plot and bad acting (Stars Wars Episode I for example), no problem for me if it looks good.
Not that I'm saying that Sin City had a bad plot, it was quite great, but the beauty of the art direction is what made the film for me.
Here's a cover of Frank Miller's Sin City comic, upon which the film noir look of the movie was based.
And now for some magnificent, dark, foreboding film stills