I have mixed feelings about the book myself. I saw it in the shop, was intrigued (and loved The Stand) so decided to buy it.
It is fantastic in the opening third, I really empathised with the characters, and found their actions believable. However:
I was really looking forward to the apocalypse hitting. You know it is coming, as the plot exposition about the weapons lab where The Twelve are being created moves forward. When it comes though, you do not get the gradual disintegration of society as per The Stand, where you get the little vignettes of people unwittingly spreading the superflu. Rather the protagonist and young Amy know it is coming, and hole up accordingly. The descent into armageddon is then relayed by trips to a local store, newspaper articles etc. I read the first section waiting for a gloriously told descent into the end times, but when I reached it was disappointed that you learnt so little about it. A lot of the reminiscences from the second section would have been just as powerful, if not more so, had they happened in the present tense.
The second section of the book was where I struggled the most. All of the characters you have grown to know vanish as you are catapulted 90 years into the future, and you have a whole load of new ones to get to know.
While there are some fantastic elements, there is also an awful lot of world-building which either does it for you or doesnt, and I fall into the latter camp.
The main issue I had was the sprawling dramatis personae, many of whom are pretty unnecessary in the overall scheme of things. Not only do many get killed piecemeal without really having any impact on the reader - you just don't 'know' them well enough to care.
The imagining of the camp, the defences against the virals, and the growing threat that the camp is facing, are fantastic though.
Once you hit the third section of the book, the pace picks back up again. I found the initial cast in section 2 to be too much, and when you get back to a smaller number of players I found it easier to keep track of who was who, and what each was motivated by.
The story also picks up pace dramatically in this section, before reaching an end that, while a little bit of a let down in some ways, has left me hungry for the next instalment.