There are some minor spoilers below, so if you have not read The Lost Symbol and still plan to, I will first encourage you not to waste your time and second note that you may learn more about the book than you wish to know in the review below.
The first of Dan Brown's books that I read, The Da Vinci Code, was fairly entertaining and rather clever. It was also somewhat preachy in parts and you definitely got the feeling that Mr. Brown wanted you to know that he is a terribly clever fellow who isn't afraid to drop names. But I enjoyed it.
Angels and Demons was my second read, though it is actually the predecessor to The Da Vinci Code. Angels and Demons was still somewhat entertaining, but the heavy-handed anti-Catholicism wore on me as the book progressed and the "gotcha" Hitchcockian twist at the end was fairly absurd and really stretched my ability to suspend my disbelief in order to enjoy the story. I didn't regret reading it, but I wouldn't really recommend it to anyone, either.
The Lost Symbol has all of the flaws of the other two books and almost none of their pleasures. About the only thing I really liked about The Lost Symbol was that Brown set the novel in Washington, D.C., a city that truly is full of marvels and which very rarely receives its due as a great tourist destination.
So, why didn't I like Symbol? Well, the most obvious answer, and the one that grated on me throughout, is that the writing is really quite poor. There are over 100 chapters in the book and I would guess that over half of them end with a "cliffhanger" similar to the following: "What he saw chilled him to his bones," or "Then, like an oncoming truck, it hit her." Do this once or twice and it can be an effective literary device. Do it a half dozen times, but spread them out over the course of a 500+ page novel and they won't really register as more than a minor annoyance. Stuff several hundred of them (no, I didn't count, but many chapters had more than one of these "oh my gosh" non-surprising "revelations") and you start to dread the next time someone in the book sees something or learns something "startling" that the reader is not privy to until much later. Or, to quote from this review by Samuel F. Lytal, "The purpose of a cliffhanger is not for you to realize it is a cliffhanger, but instead to compel you to turn the page, not laugh at the author's lack of subtlety."
There is far, far too much laughing at Dan Brown's lack of subtlety in The Lost Symbol.
Other annoyances: The pacing is lousy. The puzzles are rather dull. The supposedly clever people are far too often incredibly stupid. The villain is both unbelievable and two-dimensional. The big "thing that will shake our democracy and our world to its core" turns out to be rather trivial and banal.
There are a few interesting and entertaining tidbits in the book. Some of the history of the Masonic order and of Washington, D.C. is intriguing, and the presentation of the "science" of noetics is okay, though Robert Langdon is arguably the worst skeptic in the history of the world, accepting wild leaps of logic and intuition on the flimsiest of "evidence" and analysis. The best part of the whole thing is probably the whirlwind "tour" of D.C. that the book takes us on.
Yet even these niceties can only help make a truly horrible book into a fairly bad book. Which is a shame, because throughout Symbol, and most of Angels and Demons, I kept thinking "This could really be quite a good book if it were written by someone other than Dan Brown. Someone better than Dan Brown."
Unfortunately, it was written by Dan Brown, and it is not a good book. So, a big, ranting BLECH for The Lost Symbol.