I think I'm the only one to really dislike Zodiac. Since the case was never closed, the film doesn't have all that much to go. Past the first few letters the narrative loses it's way and decides to focus on fictional characters that have little relevance to pad it out. End result is a film that hasn't got enough to pad it out to even 100mins, yet it drags its dehydrated corpse over the 150mins threshold!!! For a film to justify being that long it needs to be EPIC (Godfather, Lord of the Rings, Magnolia), but this unfortunately isn't.
Read the wikipedia page on the Zodiac Killer, it's far more interesting and will save you plenty of time.