The trouble with a movie based on a true story is that it's also a movie bound by a true story.
Zodiac kicks off with a sadistic murderer doing his thing, then taunting the police with mysterious letters and cryptograms. The movie's tagline ("There's more than one way to lose your life to a killer") promises another dark crime thriller like Se7en, also by director David Fincher. This is the interesting part, but it only lasts for the first 1/3 of the movie, and then the reality kicks in-- the reality of a long winded and confusing investigation that spanned decades and never conclusively ended.
The second half of the movie felt a lot like, of all things, All the President's Men. That's another movie that's mostly a lot of investigating and talking. They do their best at trying to add a sense of tension by having Jake Gyllenhaal scrambling through evidence, banging on the police station door in the middle of the night, and always talking like he's out of breath and nearly out of his mind. It does work to an extent, and you often forget that years have gone by with nothing happening. It's probably as well executed as it can be given the source material. But when it's all done I can't help but feel a bit let down.
But of course, you have to respect your source material. In real life the cryptograms didn't actually mean anything, but it would have been more interesting if they did. In the end, they didn't even catch the killer, and the movie can't totally even commit to the guy it wants you to believe is the killer, because the guy wasn't convicted in real life
It would have been cooler (or at least more entertaining) if they took the premise from real life, then threw the facts out the window and decided to just run free with the concept... give it an injection of sensational cinematic spectacle, topped off with a head-in-a-box ending. I guess you have to wait 2500 years before you can do that.