Warning! Spoilers ahead! I've finished the six-book series, and I'm going to talk about it without trying to avoid the accumulated plot!
Six books. Around 2000 pages. And it was clear quite early on that it wasn't that great a series, even if it's Charlie Stross. Still, made it to the end!
The series is incredibly deceptive in what it's about. It starts off sounding like a fantasy novel (woman discovers she can travel to medieval world). It turns out it's all genetic and there's a big swing towards sci-fi-ness. It then pretends to be a parallel universes story about comparative development, technology transfer, the history of revolutions, etc.
Or it could be a bit of an action story. There are plenty of fights and all the rest of it, over the arc. Plenty of characters on the run. But... it's not really about that.
Perhaps it's about Miriam, the apparent heroine? She makes plenty of plans, but she never gets any traction, because other forces get in the way. The factional squabbles, do they ever really get any where? No! Why not? At the end of it, pretty much everything pales into insignificance next to the fact that the USA carpet bombs the parallel world with hydrogen bombs. Everything else in the entire story is pretty futile in comparison. And the events that set in motion the bombing happen decades before this story starts.
In other words, the real stars of the story are war-mongering nutters grabbing the presidency in the US, against a backdrop of a population panicked by terrorism. Everything else is just feints around an allegory of 9/11.
To summarise, this entire series, and its thousands of pages is most like his short stories - Missile Gap and the like, with their inevitable failure at the hands of evil forces.
Coming back down the scale, how's this specific book? It's pretty good, as the series goes. There's plenty of action, and as it's trying to finish things off, it's mostly not padding to be undone by the events of a later book. That doesn't rescue the time I've sunk on the series, but reading it was a bit of a guilty pleasure.