Four stars. Recommended.
It's rare to see a calm, measured tone and approach taken in a serial killer film. Zodiac is based on Robert Graysmith's book about the Zodiac killer who killed several people in California in the late 1960s and 70s, and then taunted the police and newspapers by sending them strange coded messages.
What you don't get are tense chase scenes where the killer manages to escape pursuit by fleeing across the rooftops, or a nail-biting climax where the killer is in the house! but the hero doesn't realize it. What you do get is a thoughtful, almost documentary-like examination of the journalists (Robert Downey, Jr. and Jake Gyllenhaal, who plays Graysmith himself) and police detectives (Mark Ruffalo and Anthony Edwards) as they struggle to piece together the evidence and come up with a theory strong enough to put a stop to the murders.
At about 2.5 hours, it's not a short thrill-flick, but Greysmith's obsession to keep the investigation going translates into a plot that never sags, and stays fascinating to the end.