My Amazon review:
So mostly this is the usual rather awful Dan Brown novel, with one pleasing twist, and one piece of additional awfulness to make up for it.
It's the usual awful Dan Brown because it is basically an implausible scavenger hunt starring the annoying Robert Langdon, who is even more distracted than usual by historical and architectural trivia while he is supposedly trying to save the world. The female protagonists fall for him because of course they do, and he goes on and on and on and on about things utterly unrelated to the plot (although, to be fair, the travelogue stuff is generally somewhat more interesting than the ostensible plot or the activities of the cardboard characters).
It is awful because (perhaps sensibly) the editors don't seem to have bothered editing the text (why bother when it's going to sell a zillion copies anyway), and while words like "unstraddled", "faceup" and the endlessly-repeated "bloodred" might be amusing if this was an experimental free-verse poem or something, scattered around in the otherwise flat and conventional prose they are just distracting and illiterate (would it have been so hard to type "dismounted", "face up" and "blood-red"?). He uses "enormity" to refer to a statue being large, just like a high school kid, and no one corrects him. The book even has "telegenic effluvium" for "telogen effluvium", which is embarrassing just to read, but I'm willing to assume this one is just someone not bothering to double-check a computer spell-checker.
It is awful because he gets his name-dropping quotes of Oppenheimer and Marx freaking wrong (and the correct versions would have worked so much better), which inevitably makes the reader wonder if there are equally sloppy mistakes in the travelogue and art-history sections, which would be a pity.
It is awful for the usual inexplicable references to specific irrelevant brands and people. "Maurizio reversed the boat's Volvo Penta engine, expertly backing away from the bank." (Targeted at those readers whose first action on getting onto a Venetian gondola is to check out the make of the engine) "... already skimming across the lagoon in a futuristic black tender -- a Dubois SR52 Blackbird..." (because the atmosphere would have been completely different if it were, say, a Windy 8M, a Novurania Launch 600, or heaven forbid some kind of ChrisCraft.) "Monteverdi, Liszt, Wagner, Tchaikovsky, and Puccini composed pieces based on Dante’s work, as had one of Langdon’s favorite living recording artists -- Loreena McKennitt" (Because of course we are endlessly fascinated with Langdon's every music preference and clothing habit; don't get me started on the heavily symbolic-of-nothing Mickey Mouse Watch he wears.)
It is awful for reasons that I could bore you with for quite some time (the ellipses! the completely implausible reactions to things! the dumb things that supposedly hyper-intelligent characters say! the painfully ignorant throwaway statements about what "Darwinists" believe! the more or less unchallenged and far from correct statements about how overpopulation is going to kill us all!).
It is redeemed somewhat by a large twist somewhat more than halfway through, that I admit I didn't see coming at all, and that gave me that few minutes of delight in thinking "wait, but then..." and "oh, so that's..." and paging back through the book to see what it actually said in various passages that now have completely different meanings post-reveal. The twist made a couple of things that had seemed weird and wrong on first reading make perfect sense; also a good and pleasant feeling.
But then, the crowning weirdness, that I can imagine feeling right and somewhat satisfying in a different book, but for me utterly deflates this one, is that (mild spoiler warning) it turns out at the end that everything all of the characters have done since the first page of the book has been for nothing, has made no difference at all. The world would have been just the same if they'd all woken up to the big serious threat by the Bad Guy, and thought "ah, to heck with it" and turned over and gone back to sleep (aside from the more or less indirect and accidental deaths of a couple of minor innocent characters, and some serious traffic problems in Venice). So, I mean, what? It really doesn't matter at all that Mary Sue Langdon figured out the faux-clever clues before the bad guy's deadline? So... why did I read this book, exactly, then?
It's possible that this is Book One of a series, and that in some sequel it will all turn out to have mattered. But if so that sequel will be yet another awful Dan Brown novel, and really, is it worth it?