The majority of the negative reviews on here - in particular the WONDERFUL review by A Nailor - kudos to you, that is the best review I've read on Amazon - aren't entirely wrong. I can completely understand why people are saying that the language, the characterisation, the plotting are all slightly... well, off. This has been true to a greater or lesser extent of all his novels since Monstrous Regiment, and may be (I'm really not sure) a result of Terry having to accommodate the use of speech recognition software in dictation of the novels. Certainly, they are very different animals from the earlier novels, which are much easier to read and full of snappy dialogue and splendid jokes.
So why am I giving this 5 stars? I certainly struggled through the first hundred pages, and felt my heart sinking more and more at the long and convoluted sentences, and the rather jarring scenes which seemed to have little to do with the plot.
But then, something just clicked. I slowed down my reading (and in fact went back to the beginning and reread it with a different mindset). Yes, it's not the same old Discworld, but underneath that it is still the product of the superb mind of Terry Pratchett. It took a lot of effort, but I could see what he was doing, and began to appreciate it. The humour is still there, if not so obvious and instantly accessible.
There's less overt magic, which as a fantasy addict I regret, but this is a grown-up Discworld, where magic is gradually giving way to the increasing industrialisation of Ankh-Morpork.
Do I miss the old Pratchett? Yes, of course. But this is a new phase in the developing world, and I'm glad that Terry Pratchett is still giving us valuable new insights into human (and other species) behaviour. Long may he continue to do so!