Firstly there is a lot to like about the concepts in this book:
alternative universes:

-strange, but strangy familar, new worlds (each identical to Earth but with different patterns of Evolution);
-airships (always a plus in my opinion);
- The unique 'adversary' which is not a typical evil thing but simply a form of life with the desire to popagate.

However too little is made of these and the potential wonder of new worlds is lost amongst the monotony of the 'voyage' part of the book. Which is a shame as the idea of exploring worlds with parallel courses of evolution (like the sea of mathematical possibilities described by Dawkins in the Blind Watchmaker) is fascinating and has huge potential.

Both authors' signature touches are present: Pratchett's whimsical humour and Baxter's tendency to contrast human struggles
against the very large scales of space or time. However, neither work as well as they generally do, mainly due to the difficulties with the structure and balance of the book. I would hate anyone to judge either writer's work by this well intentioned but flawed collaboration. I bet they had a great time writing it and hope that next time more of that joy can be conveyed to the reader. Better luck next time guys!