Is the violence in the 'comic book' style or in the live action sequences? I'd read a review in the NY Times today, and they didn't think much of it, but I didn't read the nitty-gritty of it. I'll go back and do that before I decide whether to take the kids or not.
Well... it's all live-action; the CGI is used to make it look "comic-booky". For instance, all the color is drained out of the film to make most everything black and white, except for certain objects: a cherry-red Cadillac convertible, a blonde in a blue dress, etc. The blood and blood-spatters are sometimes black, sometimes white, but most often a sickly neon yellow. Some of the more violent scenes (a man shooting himself in the head, for instance) are shown in white silhouette on a black background.
Before you go, I would recommend stopping by a comic speciality shop, or a Barnes & Noble, and taking a look at the Sin City graphic novels. The three that the movie is based on are "The Hard Good-Bye", "The Big Fat Kill", and "That Yellow Bastard". I haven't read them myself, but as far as I can tell the movie is very faithful to the storylines and the style of the graphic novels.