In the words of someone or other, this had BETTER be a series.
If you're expecting Pratchett, it had better be the Pratchett of "Nation" rather than of the Discworld. Which is a compliment; it means the man can write in more than one register.
At any rate, this book sits in that weird space between science fiction and fantasy that I tend to think of as speculative fiction. And speculative it is. Pratchett (and yes, I know I seem to be ignoring Baxter here, but I've never read the man so I can't really think about his authorial tendencies) has never been particularly subtle and this book is very much in the tradition of philosophical speculative fiction that forces questions on you whether you want them or not. That being said, the sly wink that is Pratchett's humor, though not as blatant, is still felt throughout the pages and it makes the book feel oddly distorted, as if it does not quite into any genre because it is, stylewise, its own thing.
It was a fascinating read, much slower paced and mellower than Discworld, but enjoyable nonetheless and the way that both the science and the people are handled makes this a good science fiction book for people who dislike hard sci-fi.
Now when's the bloody sequel coming out!?