You don't sell 40 million copies of a book when it is a bunch of rubbish. Nevertheless not everybody thinks the Da Vinci Code is a good book. Some "thumbs down" reviews:

Online Review of Current Biblical and Theological Studies.
"The most sweeping of all the fictitious claims in this book is the idea that the Priory of Sion has preserved "thousands of ancient documents as scientific evidence that the New Testament is false testimony". Such documents simply don't exist. This is part of Brown's fiction."
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Crisismagazine.com
"In the end, Dan Brown has penned a poorly written, atrociously researched mess. So, why bother with such a close reading of a worthless novel? The answer is simple: The Da Vinci Code takes esoterica mainstream. It may well do for Gnosticism what The Mists of Avalondid for paganism—gain it popular acceptance. After all, how many lay readers will see the blazing inaccuracies put forward as buried truths?"
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Leaderu.com
"Critics assail Brown's appeals to scholarship and history, which range from questionable to outlandish to (some say) outrageous. Yet, hot sales and fawning reviews by the press and readers alike (see Amazon.com's listing of the book and accompanying opinions) indicate that many are buying into this brew of conspiracy theory, romance novel and pseudo-scholarship."
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the-manifest.org
"I recently finished your mega-seller The Da Vinci Code at the prodding of my wife. I have to be up front-- it’s a really bad book-- not horrible, merely really bad in the way most off the rack thriller books are-- see Tom Clancy’s mind-numbing yawners. That said, in no way shape nor form do you pretend to be in the least bit literary. The book’s sentence and paragraph structures are a step above See Jane Run, and the attempts at metaphor are nonexistent. The narrative is about as A to B to C as one could get. It is so un-involving that it will make a far better movie, which is a passive medium. Even better, it would be a solid standard made-for-TV movie, or an old Columbo or Matlock episode."
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