It's finally clear to me that this is a 'series' in the sense of shared universe, not a continuing plot line. (I guess I might have been served better by a little research, rather than plunging in blindly. It seems Ms. Butler started with the fifth book and added the others somewhat randomly.) In any case, this third story in the series has only the faintest relation to the preceding two.
Because of the lack of connection, this book functioned largely as a standalone novel. As such it was interesting, but not particularly enthralling. Butler gives her lead characters much fuller internal lives than those in the previous books had. They're not always very credible, though. Characters are attracted to and stick to each other with less interplay than in a Harlequin novel. While Butler does good work with the result, the initial interactions are basic at best.
Some of the interpersonal attraction is explained (vaguely) by the action of the symbiont, though how this works is unclear at best, highly unlikely at worst. In fact, even allowing for this as social SF, the symbiont is a weak construct. It has either no or a fatal effect on non-humans, yet is so attuned to humans that it can sense fairly subtle thoughts and prevent related actions. That's a pretty narrow focus for an organism from a planet light-years away, and there's no attempt to explain it away.
While the setup is interesting, this feels more like the (overly detailed) middle third of a novel than a book in itself. Perhaps Butler was leaving a gap for another novel to fill later - the story of the scouting trip might have been interesting. The ending leaves a lot implied, and suggests that Butler was most interested in the hard choices that Eli faces. Unfortunately she skims through them in the book itself, and he doesn't really struggle that hard. In fact, some of these choices practically beg for examination, but don't get it.
All in all, mildly interesting, but largely a disappointment following the first two books, that while emotionally distant, had more complexity to go with them.