I heard about this book from Orson Scott Card's book "How to Write Science Fiction and Fantasy". It is being held up as a shining example of good writing. I read it in the middle of Nanowrimo month in November.
I have to agree with Card, the writing is very well done. There is a lot implied in each sentence and you can infer much about each character by their actions. I don't know if Butler does this on purpose, or has gradually internalized the process, but the more I look at it, the more impressive it is. She is a real master of point of view and implying important points without actually saying it directly.
We see an immortal man, Doro, who has been around for more than three thousand years who wanders the planet breeding a new race. Through this book, we get to see many of the results of the breeding program. We see ordinary, we see extraordinary, we see strange and even stranger.
Doro, our immortal, finds another immortal, a woman. They have several disagreements over the morals of the breeding program and how Doro treats his seed. This tension builds over the century and a half of the story. I found the characters well developed and deep.
That being said, if you were to graph the excitement versus time on this book, it would remain mostly flat. It is a subtle book in more than language, and I found it interesting, but mostly dull.
I'm still glad I read it, since it taught me that great books are more than just nice language and even an excellent premise. There is something more that I have yet to identify.