Overrated in what sense? It's not my number one favourite, but it is a pretty revolutionary piece of animation for it's time. I think the story just has this very epic feel to it, and the acting (Japanese) is just top-notch when it comes to voiceover work, it actually rivals a lot of live action performances. Plus, tons of people have cited it as their inspiration for other works that were hugely successful (movies, books, etc).
The storyline is pretty epic, but thats because it's something stupid like 15 years worth of manga wrapped up into one movie. I've watched it once (after being told how great it is) and have no desire to watch it again. I just don't think it's as good as a lot of people make it out to be. But I will agree that the animation is top notch.
As for Sin City, I'd actually say it's one of the better comic book movies as it sticks so closely to the source material.