Sin City 2 isn't impossible to follow, but without having seen the first one you miss out on quite a few nods and depth to characters. And with the nearly ten year gap in between the two films, everyone involved would have had a tough time selling it regardless.
Eva Green, though. Man...she's so gorgeous and venomous that it hurts. As a film it feels more like a sort of prequel but mostly addendum to the first film. It doesn't come across like a sequel whatsoever. I didn't dislike it. Slight disappointment is the phrase, I suppose.