You should see it if you're a fan of the noir genre. This is quite possibly the noirest movie I have ever seen, and this includes actual noir like Sunset Boulevard and The Big Sleep. It is positively drenched in noir, from the black-and-white look to the hard-boiled characters, stories, and dialogue. I will give one word of warning here: the movie will feel "off" to you in the first ten minutes since it's highly stylized. But you'll get used to it.
You should see it if you're a fan of comics. Because this will let you see how it might look if you literally transferred a graphic novel from the page to the screen, which is essentially what Robert Rodriguez did. As you watch, you can practically see the panels on paper.
You should see it if you love voiceovers. Because there are a hell of a lot of them, so much so that it often feels as if you're being told a story by the characters. This is, of course, a noir convention.
You should see it if you like Carla Gugino's breasts.
You should see it if you love movies. The way that they can transport you to another world, and for two hours or so, they can so completely engross you that nothing in your own life even seems to exist, all that matters is what's transpiring on the screen.
You should see it if you enjoy a bit of the old ultraviolence. This movie is not for children. Do not take your five-year-old sister, for God's sake. But if you can appreciate guns, arrows, grenades, swords, clubs, and ninja swastikas, then you're golden.
You should see it if you can't tell the difference between a hero and an antihero, and maybe you think there's no real distinction.
You should see it if you liked how in Pulp Fiction, three self-contained stories ending up being connected.
You should see it if you can handle gore, ranging from missing limbs to...more missing limbs.
You should see it if you want to see Alexis Bledel show a little range.
You should see it if you want to see something different. Not only visually, but stylistically as well, down to the narrative.
You should see it if you want to see a really good movie. Look, maybe I wasn't physically wowed by it, and I'm not feeling all giddy. But you know when every aspect of a movie, from the acting to the writing to the directing to the everything you can possibly think of, is just so spot-on you can't think of anything to complain about? That's Sin City. It's extremely solid, and possibly revolutionary. Go see it.