There is an excellent half a story in this book somewhere. The problem is wading through the other half to get there. Our two main protagonist's Mike Havel and Juniper Mackenzie are quite interesting characters. But in some ways that is a bad thing. Why you say, "aren't interesting characters the reason you read a story?" Very true, but with any novel based on fiction there is a suspension of disbelief that an author must ask of his readers. Sometimes credibility is stretched and sometimes it is shattered - much like stepping on a tourist's snow globe of Kooskia, ID.
As a Marine and resident of western Montana I was predisposed to identify with Mike Havel the character, but then I found out that Mike was former Force Recon (Sniper qualified too!), Gulf War veteran, master of the Finnish fighting knife and raised as an Indian tracker/hunter. I am not quite sure if such a person exists in reality but I am willing to go with it if the author doesn't beat me over the head with it multiple times throughout his book.
This compounds with the problem that our protagonist's very survival isn't just a matter of elite breeding and an unlikely intersection of family trees but also they happen upon expert bowyer/fletchers, horse hand/blacksmiths, and SCA guru's not to mention library's containing everything you ever wanted to know about ancient warrior societies, growing crops and mounted combat. Maybe this is necessary for an interesting story, but couldn't they just get lucky killing people instead of getting lucky knowing how to kill people?
There is also the problem with explosives, electricity, and pressurized gases. Every author does some hand waving to sell a story, Stirling backs himself into a corner with his Change and barely goes through the motions to explain it. The problem arises when fire works, hot air balloons work, gunpowder doesn't work, electricity doesn't work, and steam power doesn't work. So, burning coal to produce steam doesn't work in the same way that burning propane heats air and causes that hot air balloon to rise? I think as a reader I would allow Stirling to wave his hands and say this doesn't work, that does work, this doesn't etc. But when this is added to the string of luck of our characters on top of their already .005% of the human population backgrounds it just becomes a tedious exercise.
The final nail in the series' coffin for me was the ludicrous timeline for the fall of civilized man and the "Woah" scary bad people dynamic. First of all it is going to take a whole lot longer than a few months for people to abandon all signs of civilization and start eating each other. Will it happen? Yes most definitely, but not so quickly and certainly not to the degree of rolling around in your own filth and eating other peoples feet that is shown in the book after a month or two. This certainly is a way for Stirling to point at "baaaaaad" people which need to be bumped off in mass, but I reject that killing in any piece of fiction needs to be dumbed down in such a way.
I certainly won't be moving on to the rest of these books and depending on your tastes in fiction if you must read something post-apocalyptic I would suggest Cormac McCarthy's "The Road".