Sinful

This guy sums it up better than I ever could.

But, for the short version:
Dark
Bloody
Torturous
Utterly fascinating and fun to watch

Full of one-liners, that you don't even realize are one-liners until later because they are actually said in some context. Which is surprising, and perfect.

Three parallel stories that all touch, but never intersect.

The Sin City Top 5:
1 - innocent person in the whole movie
2 - more times that I'll see this in the theater (at least)
3 - castrations
4 - references to the evil that men and women do, in spite of God's presence
5 - minutes it takes for at least that many people to be killed at the beginning of the movie

Also, plenty of boobs, and women in lingerie firing very large guns.

I can't even count the number of bodies. It's classic even-the-good-guys-are-bad-guys stuff. But, oh boy are the good guys wonderful! You can't help but root for all three of them - Bruce Willis, Mickey Rourke, and Clive Owen - as they torture and murder their way through Sin City.