Zodiac is one of Neal Stephenson’s earlier books, and it shows. A lot of the writing style that went into Snow Crash is there, but it’s rougher. It’s hard to pick out specific examples, but the whole book didn’t feel to me that it flowed as well as it ought to have. On the other hand, the story was a decent one, and had several nice moments of chemistry geekiness that reminded me of the mathematically-geeky side trips in Cryptonomicon.
Surprisingly (at least to me), I liked the ending. I haven’t really been happy with the endings of Stephenson’s more recent books; I prefer something with a sense of closure. All three of his books that I’ve read (Snow Crash, The Diamond Age, and Cryptonomicon) had endings that felt unfinished. (Note that I don’t mind endings that deliberately leave things open-ended, but I do like to feel that the main story has been resolved.) Regardless, Zodiac’s ending did have closure, and I was happy with that.
So, it’s a decent read, especially if you like Neal Stephenson’s writing, but not really something I’d recommend going out of your way for.