Obsessive open source research, Robert Downey Jr. "acting" as a guy with alcohol/chemical problems, Chloe Sevigny still atoning for Brown
Okay, I'm not describing it right. This movie kicks ass. Real life doesn't fit the neat narrative structure of a traditional serial-killer film like Se7en, and this is a film committed to telling the truth about this police investigation and the paranoid culture that a (possible) serial killer can cause, and not the cheap thrill of seeing Gwyneth Paltrow's head in a box. Or any cheap thrills or easy drama at all. As Nathan Lee notes, "It's a film that never raises its voice because it needs to speak clearly and carefully. It's got a hell of a lot to say."
Zodiac is an absolutely compelling take on the difference between believing and proving, and the how much it can take to reach the latter point. See it. Oh, yes. See it.
e.t.a.: Russ makes the appropriate Malcolm Gladwell point in the comments -- Zodiac is a film about a man who sees a mystery where everyone else sees a puzzle.