An excellent riff on fairy tales. I'm not actually sure what to say about it beyond that. If you've read any of the discworld books, this one won't disappoint you.
I listened to the audiobook, and Nigel Planer did an excellent job, though I disliked his Magrat and Ella choices; his voices for them both made them sound dull and stupid. On the other hand, I've also listened to other Pratchett books narrated by Celia Imrie and I really disliked her Granny Weatherwax voice; Planer gets Granny just right - she's the crone without hurting your ear drums.
The plays on words are always my favorite part of Pratchett books and Witches Abroad did not disappoint (Emberella = Cinderella). I also loved the we finally saw Granny's magic in a very decisive show; I hope it won't be the only time we see it.