Okay, mostly it's just a mind-tour through infinite possible worlds. And that's fine. There's also a bit of Star Trek: The Next Generation-like advising in a supremely wise manner. And I like that, too. There isn't a war, which I probably prefer to skip, although it does make the title misleading. There is also stuff that was just plain wrong, specifically about drug abuse. (Drug use and abuse isn't more common among poor people, it's just more visible, and more frequently and harshly punished; rape is a bad plot device, but really especially bad if it's just a lazy-ass way to justify a male character's vigilante "justice"*). And while there are several very interesting and nuanced characters, there are also quite a few who are pure stereotype. Although I appreciate the effort to include a major character of color, it's probably best not to play that game if you aren't going to commit, and pretending that a great big dark-skinned man from South Africa is going to be automatically deferred to, rather than targeted as a thug, is a whole new kind of fantasy.
Some interesting ideas, but the most disappointing book I've ever read with Pratchett's name on it.
*And really, the ease of moving between worlds in this scenario creates a nightmare for anyone thinking about sexual or domestic violence. The bad people have a literal infinity to hide in, or keep victims in. You really don't want to get your reader's minds set on that horrific track.