It's six, six, six books in one!
Mitchell gives us an audacious stunt here. We get the beginnings of five stories, each interrupted by the next; the sixth story, at the center of the book, is presented in full, then the endings of the stories are presented in reverse order.
Like this: 1 2 3 4 5 6 5 4 3 2 1.
The stories are set over a wide range of time, from the early 1850s to a post-apocalyptic distant future, and there are connections -- some subtle, some less so -- among them. Each story is being read (in some form or another) by a character in the next; there is the suggestion that we may be following the progression through history of a single soul, reincarnated into each new time and place.
And it's quite a variety of stories, too: a 19th-century journal of a Pacific explorer; the letters of a 1930s English composer living in Belgium; a corporate thriller set in the 1970s; the memoir of a London publisher, set roughly in the present; a 22nd-century tale of enslaved clones, set in Korea; and a far-future tale of life on Hawaii.
So is the book worthwhile beyond the technical stunt? Absolutely. Each of Mitchell's stories is perfectly in style for its time and genre; the writing is clever and lively throughout. The composer's letters, in particular, are quite funny. Mitchell has smartly placed at the center of the book the two stories which will give the most readers (at least, non-SF readers) trouble; get the readers hooked in the first 4 half-tales, and they might not even notice that they're reading science fiction. (They will, however, surely notice the heavy use of dialect that makes the sixth tale rather rough sledding in spots; not quite as difficult as Russell Hoban's Riddley Walker, but surely influenced by it.)
Mitchell gets a bit heavy-handed at the end, clubbing us over the head at every turn to be sure we've caught his theme of how the powerful exploit and take advantage of others (in the summary of one character, "The weak are meat the strong do eat."), and that made the second half of the book the weaker half for me.
But that's a relatively minor flaw in what was otherwise a marvelous read. It's an ambitious book, and Mitchell succeeds almost completely.