There seems to be a division in the people that read it before they watched it. Some of us think it was true enough to form, and some of us think it was way off. I'm going to be in the first group, because short of the changes in the ending, it was pretty dead on, despite trimmings of the side stories (Tales of the Black Freighter, The Newspaper Stand), Minutemen backgrounds, and a few things that were simplified to save time (Rorschach's getting his mask back, etc.).
The opening images of the movie should be enough to please any reasonable purist. The intent and message of the movie is the same as the graphic novel, so I think some people just went in there to hate it because it was a movie and are nit-picking.
Those that think that the movie was violent and obscene didn't take the time to learn anything about what they were seeing, and their pedestrian opinion about it doesn't really matter.
Personally, I didn't see a lot of compromise, until the ending (no calamari ending...yeah yeah...get over it), and I understand it because it's a little easier to swallow for the people who haven't read the graphic novel. There was sex and violence in the novel, so that didn't seem gratuitous to me (minus Archie's flame-thrower going off, and that was humorous). Jackie Earle Haley completely nails the Rorschach role, and J.D. Morgan nails the comedian. The rest of the cast is good enough, and I have no complaints there.
Easily one of the better adaptations I've seen.