I'll break it down bluntly in a minute, but ultimately Seveneves, while being an interesting, and at some points riveting novel, is a disappointingly traditional addition to the Science-Fiction genre. 
 Technically speaking, it's a well-told story. Neal Stephenson has a strong grasp of the timing and tension required to bring the plot together. Opportunistic events do not come out of nowhere. The "gun" is always shown before it is used, and when a "gun" is shown, it is certain to go off (though not necessarily in the typical manner). 
 As deft as the plot is, however, the writing does suffer from certain pitfalls and conventions common to Science-Fiction as a genre. These include, but are not limited to, the relative 'woodenness' of characters, a typical lack of character development in favor of the larger plot, and exposition. In Mr. Stephenson's case, we are talking PAGES of exposition. 
 As I mentioned before, Science-Fiction gets something of a "genre-pass" in the telling vs. showing department by nature of its subject matter and history, but this same convention may make the writing inaccessible to readers not accustomed to it, and can be wearing even if the reader is so accustomed. Just a heads-up for non-SF readers. In hard SF, the tech isn't a distraction from the plot, it IS the plot. ;) 
 The plot itself--particularly the first two parts of the book--are a riveting race against time, characterized improvisition and the gritty reality that space is not sterile and clean. It's shocking and fantastic and I enjoyed(?) the read. Each hardship takes its toll on the dwindling numbers of humanity-in-space, until the end of act 2 brings everything to a head. 
 This is where it gets complicated. 
 The last act of the novel hinges upon a rather fantastic proposition... which is kind of a handwaved 'science-made-it-happen' scenario/object/etc. It's not really the novel's MacGuffin (that would be the moon blowing up) but it's still pretty out there. In this case, it's kind of a three-part proposition. 
 (view spoiler)[1. Seven women, with the aid of genetic engineering, are able to propogate a viable, self-sustaining, specialized, heterozyotic human population with nearly zero inbreeding issues. 
 2. The MacQuarries, from an unknown initial population (probably only a few hundred at most) were able to so the same thing, underground, in a cave, with no specialized tech, through selective breeding, and sustained themselves at the couple-hundred population size for 5000 years without problems. -.-' 
 3. A submarine culture was able to do the same thing as the previous two mentioned populations, but with an unknown tech level. Given the implications of the epilogue, I'm inclined to assume they had some kind of lab to work with simular to the spacers, unlike the MacQuarrie clan. (hide spoiler)] 
 Of the three points, I have the greatest problem with point 2. Point 3 bothers me because it's introduced so late, but if I'm accepting the rest of it I might as well swallow that. The first two acts of the novel condition the reader to accept point 1, so it's less of an issue. 
 The survival and return stories play on older archetypes in Science Fiction, and also human culture as a whole. The (view spoiler)[Seven Eves (hide spoiler)] and resulting tensions recall (in history) the Twelve Tribes of Israel or (in SF) the colonials and cylons of Galactica. The tech N.S. uses is similarly old-hat to SF lore. Even the narrative, "how could we survive as a race of spacers" has been reimagined and refined for decades as our knowledge about the universe increases. Doesn't make it bad to continue reimagining it--we keep doing it for a reason. ;) 
 I liked it. I might go so far as to say I adored the first two acts of the book. (Gory tech details and all. I like hard scifi). The third act was a little harder to get behind. Once we were focusing on people directly, it felt like something was missing. Part of it (a large part) relates directly to the 2nd part of the proposition above, but it's also hard to understand why humans--(view spoiler)[even genetically specialized ones (hide spoiler)]--wouldn't have shifted out of a certain cultural baseline over 5,000 years. Given the length of the novel and the number of asides, perhaps he simply ran out of pages to explain. 
 Also, I take serious issue with the mechanics of the Cradle, but N.S. flat out said in his author's note that he was ignoring it this time around, so *shrug.* 
 I still liked it. Could have been better, but it also could have been a million times worse.