My one criticism of the final book would be that there's a lot of philosophical chin-wagging, probably a little more than necessary for my taste. While the series is obviously largely about the kinds of sacrifices we can, or should be prepared to, make in order to "save the world", some of the issues in this book (I think more so than in the first and second books) are presented in "talking heads" style dialogue between characters, and the conversations become a bit repetitive. There's a lot of conversations about: "How far would you go to save the species/save the world/ensure a future for the planet?"
The book also covers a lot of physical ground with bands of characters traversing the country and the cities in order to achieve their goals. I would get close to saying there's too much going on in this respect, although it's never difficult to follow. But at times I did wonder how the heck we were ever going to get to a resolution of the story with so much going on, including the introduction of several major mysterious new characters.
In fairness, Wells DOES conclude the story in a satisfactory manner and ties up all of the loose ends. He gives a sense of a directed, if shaky, future for the planet which is well in keeping with the story.
Wells is obviously a master of spinning a complex tale and constructing detailed and touching narratives for all of his characters. I grew to really enjoy this series and to feel strongly for all the characters. So I'm a Wells fan now undoubtedly. I highly recommend the audio versions of all of these books in particular because the narration is really top notch.