This is such a well-made film in every detail. The cinematography is great, the acting ,even by those who are on screen for only minutes, is pitch-perfect, I loved the period details, and even the small character moments work. Despite the jumping structure, the film never feels episodic, and I was fully engrossed. And I liked the point Fincher made by having three different actors portraying the Zodiac killer: you might think you know something about who did it because you see the murders happening, but really, you can't even tell if it's the same guy.
So, a very well-made film. A great film? For that, I don't know if there's enough under the surface. Sure the film's about obsession, about making the viewer as obsessed as the characters are, but it doesn't really tell you anything about what would make someone more susceptible to obsession than another, about what it's really about.
I wonder what Fincher could do with a costume drama, a romantic comedy, a musical for all I care. It seem he's pretty much mastered this genre, and it would be interesting to see him go outside his comfort zone. It might make him loosen up a little. Benjamin Button is a start, and I'm curious what will come out.