I haven't seen it yet, but I've read it's not too bad even for those of us fans of the graphic novel and Alan Moore in general. Personally, I didn't like the "updating" of the costumes too much (they don't look 1985 anymore, where the book and movie are set) and the Comedian looks way too young and needs to eat a sammich. However, it may be a good movie by itself (or an OK one, like V for Vendetta), I'll have to see it.
About the three-dimensionality of the characters: it's an unfortunate casualty of the format. There's no way to include all the backstory subtleties that the novel brings in a 3 hour movie, it would need a miniseries. Moreover, the amazing subtleties of things like Dan Dreiberg's ornithology column name for example ("Blood from the shoulder of Pallas") will probably never translate well to film.