Like most book adaptations, this one is never going to be as good as the book. Characters and plot threads are entirely missing, we never learn the rat language, and the pied piper's arc of the story is entirely rewritten into something with none of the wit and charm that Pratchett wrote the original arc with - among other issues.

As a movie it works, and it draws many things from the book and follows the major plot thread from start to end. The dialogue is mostly lifted straight from the book, even. ... As long as you don't overthink the plot and go "wait, did we just skip something?"

As an adaptation, I find myself rather disappointed. Dangerous Beans is reduced to a rambling mess of a character that makes no sense (and doesn't appear to be blind), and Hamnpork's character was forcefully merged with Darktan in a very clumsy way. Several moments key to the book - like the rats forgetting how to speak at one point - simply don't exist in this version of the story.

Watch it, but don't expect perfection.

I've not tagged this for spoilers, as the spoilers are largely for the book version of the story and the movie takes a different course.

Read the book. It's more coherent, more charming, and has a better second half.