Oh, I've seen violent films, just most of them (especially horror films) tend to be obviously designed as entertainment.
Sin City, on the other hand, seems to be nothing more than a straight, bleak presentation of death and gore.
I failed to identify or empathise with any of the characters, and to be honest, I don't think any of them had any good or redeeming features. The portrayal of women - in particular - hit hard, and I spent most of the film in shock.
That people somehow take enjoyment from watching such base imagery baffles me, and I think it's fair to equate it to snuff, as you are deriving pleasure from brutality.