The plot is deceptively simple - on the first page, the moon explodes. We don't know why or how, but we do learn very quickly that it spells extinction-level-event for the people of Earth, and the book follows the way the human race deals with this new reality.
What I loved about this book is that it was straightforward without losing its complexity. It's hard sci-fi in a lot of ways, and the amount of science and theory Stephenson jams in here is pretty great. Why this worked for me, though, is that the politics and social activities that surround the disaster and beyond felt extremely real. While it's hard to give away crucial plotpoints in what is a nearly 900 page book, the fact that the book kept me so engaged on a lot of those issues ended up being a really pleasant surprise.
Another benefit of the story is how things came around toward the end, and how so many quality payoffs occur. One specific incident toward the end had me pounding the book in excitement, which never happens, and really brought the whole thing around for me.
I guess, if I have a warning, it's that this book won't lend itself to falling for specific characters. Just don't do it. You'll be better off. Instead, enjoy the long-term ride that this provides - it's a different book than Anathem, concerned less with philosophy than with technology and science, but still balancing things out in a good way. It might even be a better book than Anathem, even though Anathem appealed to a lot more of my base interests.
If an almost 900 page epic science fiction tale isn't daunting to you, get your hands on a copy of this immediately. This is, without a doubt, the best book I'e read in 2015 so far, and it's not even close. Highly recommended, a great ride from start to finish.