Probably closer to a 3.5, but I'm in the mood to be picky.
It's a good book. It's surprisingly straightforward for a Mitchell novel (so weirdly roundabout by everyone else's standards) and, as others have observed before, this may be his most speculative work yet.
Still, I did find that the Mitchellness of the writing did get in the way of the story. There were all these wonderfully written vignettes about life and people and, in the background, this larger and stranger narrative building up; a narrative that gets about 100 pages in the spotlight before fading away. And I kept either wanting the larger narrative to pick up the pace and explain what it was doing or, alternatively, get out of the way and let the focus go back to the individual characters, who were awesome. The story as told through multiple people is a staple of Mitchell's work, but this story is not always well served by that narrative. He doesn't marry the large scale and the small scale as well as he did in Cloud Atlas and that unevenness does detract a bit from the book.
Also, mildly spoileriffic pet peeve, how is it that the man cannot imagine a future for our world in which society does not collapse? I just...it's every book! And it's not even the point of every book. It's just a thing that happens!
Anyway, still a really good read and the writing remains gorgeous. But the book itself feels a bit rough around the edges, as if it's not sure how to be literary and genre at the same time. And I'm not sure why, given that Mitchell has done a great job with this combination before. But it just fell a bit flat at times here.