Probably the most faithful adaptation of a book to the screen that I have seen. The animation isn't the best, the transitions are a bit strange, and the dialogue is very fast as if they tried to pack every line of the book in (they nearly succeeded). However, it is still a delightful rendition. Voice acting is top notch. Christopher Lee as Death, as is traditional. It's wonderful to see Granny Weatherwax, Nanny Ogg, and Magrat Garlick come to life.
I've heard some people say that if you haven't read the book before watching, you won't be able to keep up with the jokes and plot. I watched it before reading the associated novel (though I had read several other Discworld titles) and had no problem. I keep going back and watching it again! Which isn't common for me with a movie. I discovered it about a week ago and have watched it about 6 times since then.
Even better, it made me aware of the Witches storyline in Discworld. I love Sir Terry Pratchett's writing style and I'm also a huge fan of High Fantasy. The Witches series has quickly become my favorite Discworld storyline. I can't get enough of Granny Weatherwax and Nanny Ogg.
I imagine because the series is out of circulation, it is available in its entirety on YouTube. Easily found by searching the title.