I am a Pratchett addict; I took my first dose in 1976 with 'Dark Side of the Sun' and have read everything he's written since (and went back to read 'The Carpet People' and Strata') I read the Nome Trilogy - I even bought 'The Unadulterated Cat'!
For a time in the 90s he spoiled me for other fantasy writers; his style was (to me) so accomplished that others couldn't begin to match him. The earlier Discworld books, from #01 'The Colour of Magic' to #10 'Moving Pictures' were wonderful romps with a hugely imaginative drive. 'Equal Rites,' 'Wyrd Sisters' and 'Guards! Guards!' were the absolute pinnacle of comic fantasy.
Later books (with occasional returns to the earlier broad humour) were darker, more thoughtful and with a more philosophical edge. Gradually the humour became less important to the story - the books were still funny in parts, but the Discworld became less magical and more a distortion of our own world, tackling in more detail real issues such as class, racism and sexism - prejudice in all its ugly forms. He even created a new 'ism' - speciesism. He was at his best in this period when he was angry about prejudice in books like 'Small Gods,' 'Lords and Ladies' and 'Feet of Clay'.
Even the 'lesser' works (again, to me! I know it's subjective) such as 'Soul Music', 'Hogfather' and 'The Last Continent' had enough of the classic Pratchett mix of wisdom and gags to satisfy most of us.
I feel the last great Discworld book (for adults) was 'Thief of Time'. The last great book for younger readers was 'The Amazing Maurice and his Educated Rodents'. Both of these were classic Pratchett, filled with imagination and brio.
And then... things started to change. The Tiffany Aching books were, increasingly, becoming just a little less vivid and gripping in their execution. The Moist von Lipwig books were pretty good, but a little too long and under-edited. 'Monstrous Regiment' was (to me!) muddled and the characterisation was weak. The lightness of touch that characterised earlier books was gone; the moralising in the stories became more blatant and heavy-handed.
'Unseen Academicals' - well I hate football so maybe some of the humour went over my head. Not bad, but not great.
Now, this book... well, it's ok. I like Vimes a lot, and thought he acquitted himself well in 'Snuff'. The plot was as usual fairly convoluted but worked well. The pacing was a little off - some passages went at a snail's pace, others seemed rushed and incomplete. Young Sam is a great character, developing nicely, and it was nice to see Willikins in all his bruiser glory.
The dialogue is the greatest change in Pratchett's style - where it used to zing off the page and allow you to really identify with the characters, it now seems a little stilted and over-complex.
Having said all this - even a substandard Pratchett is considerably better than most other writers' masterworks... I will always buy a new Terry Pratchett book and take what enjoyment I can - and there is always a great deal of enjoyment, just a little less than in the past. I still must have my regular Pratchett fix...