Just finished the Long War, the new Terry Pratchett/Steve Baxter collaboration.
Confession: I am a Pratchett fanboi, and I also thought Good Omens - his collaboration with Neil Gaiman was one of his best books.
When it plopped through the door, I suddenly realised I had missed The Long Earth, the precursor. Only one thing for it, whip out the Kindle. This does make it the only Pratchett I don't (yet) have in print. No spoilers follow.
The two were not overly dissimilar, as you might have expected. Both felt as if they were constructed to be a series as is often the case with SF world books. You can feel Pratchett's hand quite strongly in the construction of the characters and the humour, but the two write quite well together. I romped through the Long Earth, but the Long War could (in my view) have benefitted from stronger editing. There is a much better sense of narrative in LE as well, LW is a little confused - too many characters, too self consciously woven together, too much chopping and changing from chapter to chapter. The LW has a better.
All in all, an interesting combination of fantasy & SF, with enough science plausibility within the confined of the imagined world to be reasonably SF satisfying, combined with the anarchic humour if somewhat cardboard cut-out characters that are intrinsic to Pratchett. Reading reviews on Amazon, there are fans of both Pratchett and Baxter who dislike the result, it has more to do with their broadness of vision than the quality of writing.