The film adaption for Sin City came out when I was I think a freshman in high school. This was before I got into Frank Miller, so I hadn't read this book at the time. If you're going into this read cold, I think you will enjoy it as a classic-yet unorthodox-black and white noir piece. Marv's narrative voice was strong and consistent and holds up the gritty and(ironically) colorful world that is Sin City.
IF, however, you have already seen the movie, know this: the movie is frame for frame, shot for shot, exactly like the book. Granted, there are some details in the written version that better flesh out Marv as a character and provide some interesting insight into the city's history, but they don't alter the story itself. There is I think one scene that is in the book that is not in the movie, but if you're like me, then you've already seen it in the DVD extra.
Sin City is a great addition to your comics library and a great start for anyone trying to get into comics. The writing is slick and the artwork is striking in how successful lighting and negative space is used. Looking forward to Vol 2!