Once again, the great strength of the series is the wealth of ideas: an infinite number of Earths, and now, an infinite number of Marses, as well. That part is fun.
Weaknesses: well, there's not a lot of opportunity for character development, the book is distinctly British, even though it's primarily about Americans, and the sudden development of an advanced form of homo sapiens is hideously implausible from an evolutionary standpoint.
Strengths, again: well I really enjoy seeing how conflicts are settled without weapons. And I like the talking cat. But when I think sadly of no more Pratchett books to come, I won't be thinking so much about this series. They're better than average, but they aren't great.
Library copy