I would classify this as an epic sci-fi tale. Like most apocalyptic stories, it is depressing and most of the characters we've been introduced to die. But unlike most end of the world stories, instead of focusing on the good still left to be found in mankind, this one often focuses on all the evil that comes out in force during crises.
The most annoying part of this book for me was the non-linear chronology. As an epic tale, we get the story from many different POVs. But each time the POV changes, we are given long flashbacks included in with the "current" timeframe. Because of this, it is almost impossible to fit the pieces together into some reasonable notion of what-happened-when.
Although we expect to be introduced to many characters due to the length and the epic nature of the story, the sheer number of characters made the story confusing. It was also hard to determine who was an important character and who was not. Many characters were introduced strongly only to quickly be killed or just to be dropped from the main thrust of the story. For instance, the book starts with Russ Corey, the most important man on the moon, and we see him off and on for the first half of the book, implying to me that he and the moon might be important in the story. But then the moon subplot is completely dropped until almost the end of the story!
Finally, like most books of any length, the author seemed to tire of writing near the last quarter of the book. The story gradually degrades to snippets from ships' logs and the time covered speeds up like the author is just trying to get to the end of the story. This type of ending always leaves me disgusted because it doesn't do the story justice.