The epistolary format really doesn't work for some people, but for me, it totally did. I like the story, and I like that it's not focussed on the slasher gore aspect - there's only so many ways you can describe someone holed up in a house, fighting their way out with a machete, and after the first few it's tired, so I really enjoyed putting a different spin on it.
I like the sentiment, there's some astute observation about the nature of modern society and it's similarities and differences around the world. I particularly like the fact that the entire "World" war wasn't played out in Southern California, and that most of the survivors "interviewed" are clearly suffering from various kinds of PTSD and other fallout. Winning the war didn't save the day entirely, it damaged people.
I can see why a lot of people see this book as needlessly gimicky, and lacking characterization, but I found neither of those things to be true. To each their own I guess... or you know, go tell it to the whales.