Finished reading The Rule of Four a couple of days ago. While its an interesting book, I was disappointed.
People are comparing it to The Da Vinci Code but there are only few similarities. Both deal with hidden messages in old manuscripts/books. But while the Rule of Four goes much more deeply into the characters of the book, it gives little detail about the intricacies of the puzzles. It explains the first few "riddles" reasonably well, but then just kind of punts on the rest. I remember The Da Vinci Code gave much more detail on its puzzles, their background and how they were solved. Most of the puzzles in The Rule of Four boil down to finding an answer to a riddle (one word) from Renaissance literature and using it as a cypher to unscramble text. And the scope of the book is much more limited than The Da Vinci Code, dealing only with occurences on the Princeton campus (and the past life of 3 scholars) instead of Da Vinci's globe spanning story.
I guess that if people want more of a "personal" story, with interesting characters, then Rule of Four is probably pretty good. But if like me, you are more interested in the puzzles and the details behind them, then its disappointing.