I love Terry Pratchett's books, having read each one an average of 3 times, and finding more funny stuff each time. I regard him as the most talented (living) satirist in the English language. Although his most famous novels are set in the Discworld, they are meant to have resonance in the modern world. And they do, spoofing Hollywood, the space program, governments everywhere, and us. The books on which this movie are based are early novels, with a bit less nuance than the later novels, but still outshine the movie.
Its main problem? It's too long. Some judicious editing is in order. The incidents are funny and well set, but everything drags just a little. Loved Jason as Rincewind, the tourist and his luggage, and cameos for Vetinari and Cohen the (retired) Barbarian. But with all the successful bits, there's a little too much exposition, sections where little happens, and it becomes a little tedious. I would like to love this movie, because I love Pratchett's written work, but it doesn't quite meet expectations. Hogfather was better.