If you've read any of James Rollins' previous books and hated it/them then I wouldn't bother picking up The Doomsday Key. This latest adventure featuring the men & women of SIGMA follows the same well worn template used for all of Rollin's previous books; namely some violent incident, this time involving the death of a priest in the vatican, leads to SIGMA uncovering a hidden conspiracy and at the same time solving some long hidden historical mystery. Stir in some pseudo-science to give it a pretence to factual accuracy, a bunch of treasures guarded by Indiana Jones-style traps and a series of action set pieces and you end up with a piece of OTT, often ridiculous but none-the-less hugely entertaining action adventure hokum. Nothing more and nothing less.
Assuming that you're a fan of such things and of Rollin's earlier books you'll probably find that The Doomsday Key, despite being entertaining, doesn't quite live up some of his previous efforts. Whilst it ends with an rapid fire, tense finale, the pace of the first two thirds of the book is slightly off and at times the story seems to slow to a crawl or meander badly. The central mystery isn't quite as compelling as it needs to be either and the main sub-plot, involving genetically modified crops, never really catches fire and then simply peters out. There are some great set pieces, such as a blizzard bound fight in the artic reaches of Norway, but they aren't enough to entirely rescue a book that needs to be punchier but gets weighed down with too much exposition.
Rollins also needs to move the whole 'Guild' story arc, that has run since the first Sigma novel, forward. I struggle to remember exactly what the Guild's primary motivation is at times. Its all very well keeping them as an ominous, mysterious, secretive and ruthless threat in the background for a few books but if we don't begin to find out more about them, how they're organised, who their members are, etc I'll begin to lose patience. For the threat to remain palpable they need to be more than a name and a bunch of expedendable mercenaries who get their comupance at the end of each novel. The Doomsday Key sketches in a few new details about SIGMA's main adversaries but leaves just as many questions unanswered.
So, if you're a Rollin's fan you'll no doubt enjoy The Doomsday Key despite its flaws. If you're new to the author I would recommend picking up Sandstorm or one of his earlier standalone novels before tackling this latest one. If you haven't liked any of his previous books then you're unlikely to enjoy this one either. Personally I liked it but I also know he can do much, much better...