Terry Pratchett and Stephen Baxter are about as unlikely a pairing of SFF authors as one could imagine, the one ever lighthearted and focused on comedic fantasy, the other dour and with a manic grip on manned exploration of space. I enjoy them both, and was curious to see what this conjunction would bring.
Unfortunately, the result here is not quite what one would hope for. Pratchett's manic humor is tamped down, and Baxter's cold narrative is humanized, but the book lacks any real spark. And while in most collaborations, one author writes the draft (here I'd guess Baxter), and the other one edits, here it sometimes feels like they co-wrote some of the clumsy sentences in the opening chapter word by word.
While the environment is interesting, the characterizations and interactions are almost entirely from stock. Very little happens that is really surprising, and some things happen purely for dramatic effect - and we can see it. More problematic, the book makes no real pretense of being a stand-alone novel, though it could easily have been one. Instead it just winds down and then gives up - a truly anti-climactic end after a very mild resolution of what was built up as an exciting mystery.
Both of these authors are good writers on their own, so (after the beginning), most of the writing is good enough, but there's not much new here. Standard alternate world scenarios move along, but don't benefit from either Pratchett's wit or Baxter's painstaking care.
All that said, it's interesting enough that I could see getting the second book, if it's cheap. If you're looking for an exciting collaboration, this isn't it. But if you have an interest in a decently told alternate world series, go ahead and pick this up.