This is superlative fiction, whatever genre you try to put it in. I came to the book knowing nothing of the plot, and that's as it should be - otherwise you miss some of the most interesting surprises of the book. The reviewer who entitles their review 'the moral responsibilities of aliens' (whilst giving a good review, not that I agree with all of it) gives, I feel, far too much of the plot away. So if you're one of those people who, like me, don't like being told the middle of a book before you've even started it, then I have to say be careful reading such detailed reviews. I liken it to being told the plot twist of 'The Sixth Sense' - it's just 'not done', is it chaps?
Tepper always deals with broad moral themes, but you never get the feeling she's preaching. Her characters remain as vivid to me now as when I read the book last (which was quite some time ago). I won't go on, because better reviewers than I can cover this. My enthusiasm for this book renders me a little useless for rational dissection.
And if you, like me, love 'Grass', my next recommendation of Tepper's work would be 'The Family Tree'. And don't read any reviews, because if anyone gives away the big plot twist in THAT one they deserve to have every new book's plot explained to them in detail before they open the covers for the rest of their lives.