Very strong apocalyptic/ post-apocalyptic fiction. By structure it's two or three novels and has the length to match. With different choices it could be edited much more tightly without touching the core plot. Given the subject matter it's not a surprise that large numbers of characters are introduced only to swiftly exit. Locations as well; the book spans much of the United States in one time or another and the ruined world is well depicted with little details like accumulating earthquakes destroying roads near California or no one ever removing mummified bodies from where they fell. No punches are pulled on who dies in an apocalypse. The book has quite a few dead and implied deaths of children.
The threat itself is interesting. They're fundamentally infectious vampires and are the worst of both worlds of a zombie apocalypse. There are a lot of them, they have animal rationality and they rarely halt an attack for anything but they are all individually very dangerous, infectious, and difficult to kill. The reader is aware that there is more depth to the problem than most of the characters know and it does explain why the lethality and fighting skills of the "virals," while never low, can vary according to plot needs.
The book tries to cover a lot of ground, both genre and subject matter, and more technically minded readers can find flaws with the world building or some specific rules about the "virals" and two of the main characters. Characters are able to salvage vehicles and fuel after a hundred years of abandonment. Characters who know that immortal monsters are a fact of life and used to be human don't recognize a teenage girl that has conclusive evidence of being a hundred-and-teen, doesn't like sun, heals rapidly and can walk through deadly territory unarmed is, well, probably a vampire. Similarly how her blood and the virus work appear to have gone through a few drafts. It's a significant detail at one point that multiple characters are splashed with her blood from an injury but they neither get ill nor clearly develop powers - although one's fighting abilities perpetually border on superhuman.
The ending also, while being aimed at through the book, comes on very fast and neat for a work that spans around 700 pages by that point. There are some nods to causality-bending visions that are never explained as part of the reason for this, but it's still mechanically "neat."
Overall these are minor issues in an otherwise strong book. The book also finishes strong enough that while sequels are implied (and do exist) they're not necessary reading like so many wannabe trilogies tend to be.