Every so often, a science fiction novel comes along that transcends science fiction. A novel that, while clearly genre fiction, can be read with pleasure by any intelligent reader. A novel that is not aimed only at those of us who grew up with Asimov and Heinlein and Niven.
Seveneves is not that novel.
Actually, Seveneves is sort of three novels, none of which are that novel. The first two are tightly coupled; the third is not. All the parts have large quantities of technical detail--don't even think about reading Seveneves if you didn't like Apollo 13 and The Martian. The first two-thirds contain large infodumps about orbital mechanics, while the last third is heavy on the details of Stephenson's imagined future society.
Stephenson being Stephenson, i.e. a superb writer, all those infodumps are very interesting. This stuff is like intellectual crack for the right sort of reader, which I am. There are elements of serious classical Hard SF here: I see traces of Ringworld and The Moon is a Harsh Mistress and Kim Stanley Robinson's Red Mars et seq. (which last I actually thought were awful, but that critique does not apply here). I don't mean to imply that there is no characterization or plot or theme: the book is 867 pages long, which leaves plenty of room for everything. But the Big Ideas, and the precisely-worked-out Big Engineering details, are not the least of its pleasures.
Seveneves as a whole is very good, for those of us who are within the bull's-eye of its target audience. I think, though, that it would have been better as two books, for what that's worth. The first two parts are one story--a mixture of space story, disaster epic, and political thriller, with a lot of narrative tension. The third portion is a completely different story, and a completely different kind of story. In fact, for much of it, it's not quite clear what that story is; it comes near to being overwhelmed by the descriptive text. If you are among those who feel that Neal Stephenson's weakness is in coming up with good endings, Seveneyes will not change your mind.
I liked it very much. Whether you'll like it depends, more than anything else, on what sort of reader you are.
If you haven't read any Neal Stephenson before now, I wouldn't start with Seveneyes. Give Cryptonomicon or Snow Crash a try. Be warned: Stephenson has many different authorial voices, and the one he uses in those two books is quite distinctive (and often very funny).
If, on the other hand, you'd rather read the novel that Seveneves isn't--the SF novel that isn't aimed at SF junkies--try this one.