This was my first, and long overdue introduction into the dark and often schizophrenic world of Chuck Palahniuk. I read this in September of 2006, so you'll have to pardon the lack of specific details.
Palahniuk's success, or at least, my appreciation for this book, comes from his darkly paranoid writing style. In many cases, authors who write to imbue confusion in the reader do so at the expense of narrative cohesion. Diary's success lies in the fact that it disorients the reader, but can describe in parallel separate lives and disparate events as part of a cohesive narrative thread, while maintaining the uniqueness and economy of prose that Palahniuk brings to all his books.
That said, Diary is not a book for the faint of heart. Palahniuk's oft-schizophrenic prose imbues the reader's mind with unsettling, and often frightening imagery and emotion as he digs into the familiar territory of identity and its social constructs through some very interesting narrative conceits. Of course, it takes two to three readings to fully explore the depth of Diary, a classic trait of Palahniuk's books. While Diary does not live up to the standard of Fight Club, it is certainly a book worthy of attention for the seasoned Palahniuk fan.