The second time around, once you know the secret, it is a lot more interesting. I'm not a spoiler, so I'm not about to tell you the secret. I will say that I began to see small complexities that emerge without much fanfare, and are also missed by the main character, not knowing how significant they are to her own life. You begin to see the work the author has done to keep you guessing. You begin to understand why the main character had no idea what was happening, either.
Although a lot of Palahniuk's other books are - honestly - weirder than I usually like, I really enjoy reading the ones I've liked multiple times. For all of his weirdness, he comes up with concepts that are so imaginative and insane that I can't help but enjoy them. Diary is a beautifully-choreographed train wreck that most of us could never even dream of - much less write and furthermore, have the guts to publish.
I actually felt like starting it again when I finished it last night, just because I wanted to see what else I might discover on the third time through.
If you have an aversion to gore or explicit language, skip it.