I watched Zodiac last night. Now, I loved it, but I can also sit through a two-hour "48 Hours Mystery" and countless episodes of "American Justice". If stuff like that bores you, then chances are this movie will too. My husband nodded off several times and I had to elbow him awake.
I have not read Robert Graysmith's books on the Zodiac killer, but the movie piqued my interest enough to put library holds on them.
Jake Gyllenhaal was surprisingly effective as Graysmith - the only other time he's made an impression on me was in Brokeback Mountain. And Robert Downey Jr. does his usual wonderful job - he's at his best as a slightly effeminate, substance-abusing, cynical crime reporter (Paul Avery, who actually wrote the newspaper articles on the Zodiac killer). The movie does lose a bit of its flavour when Downey departs from the main action, though.