I'm a big fan of Frank Miller, and I'm glad that Robert Rodriguez let him in on the making of the movie, it was only right. My only gripes are since I've read all his Sin City graphic novels, I knew what every scene was pretty much going to look like and how every story ended. That being said, Mickey Rourke did a great job, and Jessica Alba actually did a better job than I expected.
Having Tarantino direct a segment sure as hell didn't hurt, either.
It's my movie of the year so far (pretty weak year for movies so far, IMO), with Constantine a distant second.