It's an easy read that is popular. I don't have a high opinion of it, but IMO it's the most readable of his books. And hey, I read it. Then tried to read two other books of his and didn't get into them.
There's two good things about it.
IIRC it's very readable with a clear simple sentence structure.
The chapters are structured to have some kind of hook that makes you want to keep reading.
It's basically an airport novel or a shelf filler for a non-reader.
Edit: Dan Brown did a fantastic job at exactly what he set out to do, making money. There's nothing deeper about human nature being communicated. The book is just a mediocre thriller with cardboard thin characters that was written in a way that made you want to finish reading it. He did a great job of making it simple for a broad audience and a great job of keeping that audience's attention held.