This book, and everything written by Dan Brown (to varying degrees), represent much of what I most dislike about pop literature. First of all, Mr. Brown, despite teaching English at Amherst College, is a bad writer. This is not to say that I am a good writer. But I recognize a person who can't "show" you vivid scenes, he has to "tell you". Various characters wear expensive clothes. How do we know? The text says they're expensive. How do we know Mr. Langdon is brilliant? The text makes no bones about telling us. Langdon is also famous. Blah.
Furthermore, Mr. Brown's books are ridiculously formulaic. Every single "thriller" that he has written to date begins with the murder of a key character at the hands of a shadowy and "terrifying" assassin individual/group. This group is controlled by a larger group with dubious intentions that generally have to do with world domination. The protagonist is introduced as an "expert" whose credentials relate to the matter at hand, and who takes the job of hunting down the bad guys. He enlists the aid of an extremely avuncular, wise, benevolent helper. This person provides assistance as the protagonist (with a love interest) finds clues to the murder, attempts to find the bad guys, is pursued by the assassin(s), all while TIME IS RUNNING OUT. The avuncular father figure turns out to be pulling the strings of the assassins, is behind the original killing, and provides a forgettable monologue at the end where he pleas for understanding. But our hero takes him down. The end. I'm sorry if I just ruined all Dan Brown's books for you.
Finally, Mr. Brown likes to write about what he sees as religious conflicts. These conflicts take place between believers and non-. Unfortunately, he proves unable to adequately and convincingly describe these conflicts, because he reveals a striking inability to understand why people believe, in the first place. His highly religious characters therefore invariably turn out to be crazed nutjobs. I don't like stories that exploit religion for entertainment, and then use the attention that they draw to this entertainment to subtly undermine the reasons for faith.
But by all means, read the Da Vinci Code. People say it's smart. Others describe it as a fast-paced thriller with historical and theological implications. It could've been in the hands of another author.