In all honesty, I did enjoy this. It's a quick read, reasonably informative without getting too boring, and fun enough.
The reason I'm giving it only 3 stars though, is because numerous areas of the story almost read like an advertisement/recruitment notice for joining the Freemasons.
Now, I'm nowhere nearly enough informed on who the Freemasons are or what they do, but I know of their existence...and that's enough, frankly, for me. In the Lost Symbol, the Freemasons read like a fun secret society that have been terribly misunderstood throughout the decades. Robert Langdon, while being his usual clever self, constantly seems to be either extolling the virtues of the masons, or defending and clarifying all their actions.
Don't get me wrong - I have nothing AGAINST the Freemasons, but I did find it rather irritating when Langdon would start to go on about them again and again. I would say that's probably my biggest nitpick of the story. I get it - the Freemasons aren't that bad! Now let's move on already!
Aside from that, I did enjoy the rest of the book. The details to Washington have piqued my curiosity of the US capitol and ensured that I will definitely be taking a closer look if I ever get a chance to visit there.
I would give it 3.5 stars if I could - a fun read, but you may want to gloss over the rah-rah Freemason lectures.