I was one of those readers who enjoyed the first couple of stories in the series. I don't recall the reason I was turned off by the last story I read. From what I do remember, the meteoroid shower had ended, the infected had died, and the survivors where disposing of the dead to clean up the surrounding area. The dynamics had changed within the small group of survivors; caused by the death of a few key members in their group if I recall correctly.
Actually, the last book switched from the main character, David, and instead followed his followers as they traveled the country trying to spread his cure. This issue with the story was the lack of a single focus, as much as I could determine. Just as the readers started liking the characters and new story, I'd switch to an entirely new locale, new people and different problems.
I knew going in that it would be difficult maintaining interest, but hoped I was up to the challenge. I really enjoyed the story, myself, but can understand why others had trouble with it. The problem was more with the execution than the idea.
The new book again leaves the lead character, David, behind, but it focuses on a central character, Natalie, as she travels to Europe to take his cure to the rest of the world. She still communicates, when she can, with David, but the attention is squarely on Natalie as she faces language and cultural difficulties as she tries to convince people to undergo the treatment and rejoin society.
However, it's still a question of whether I can pull it off, given the weak start the previous book left me with. Natalie isn't a real compelling character, as the most unsure of David's followers. Also, with the constantly changing locations and people, it'll likely be hard to follow as well. Thus it remains nearly as difficult proposition as the previous book. Thus the question, if the series has lost steam, is it worth continuing with partial fixes in perspective?