Seveneves is lengthy. If you read Stephenson you know what your are in for: highly detailed descriptions of anything and everything. Stephenson doesn’t just answer the question, he shows his work. I like that. I learn things every time I read one of his novels. It isn’t for everyone, though. People who dislike hard sci-fi, who read only to see what happens to the characters, will probably find Seveneves a slog. I do like hard sci-fi. I don’t write it, primarily because I can’t. I do admire those with the science background and aptitude to create plausible futures extrapolated from existing technology. (My science fiction is essentially fantasy with spaceships. The science might as well be magic. But you do get to see what happens to the characters, so there’s that.)
The truth is, however, that I found Seveneves to be one of Stephenson’s lesser novels. The first two-thirds is engrossing, detailing the steps involved in the survival of the human race after the destruction of the moon. The last third, taking place millennia later, was written well enough, but felt comparatively slight after the previous bit. I’d rather have gotten more details of the intervening generations. And I found his genetically-driven personality types somewhat implausible. He explained the concepts well enough (this is Stephenson, after all) but I couldn’t quite buy it, nor the limited extent of interbreeding. But it was still entertaining.
The ending, now. Well, Stephenson is part of the Stephen King school of endings — Unsatisfactory U. In a blog post, Stephenson stated that the “only part that gave me any trouble was calibrating an ending that would leave the reader satisfied that the story had concluded while leaving the impression of an open-ended world.” He succeeded in the latter. Not so sure about the former.
However, if you liked Snowcrash, Cryptonomicon, Anathem, Reamde et al, I’m comfortable recommending Seveneves. Of course, you could always wait for the movie.