I think George Guidall do a great job reading Wolves of the Calla. There has been some critic of his reading on the Internet and he has often been compared to Frank Muller who is considered to be the King audio book narrator. I must say that I don't really think that is fair, neither to Frank nor to George because they really have two different styles when they narrate a book. They are both really good narrators in there own ways and can't really be compared.
As I said in the review of the book version, Wolves of the Calla it's one of the best parts in The Dark Tower series so far so the book is pure joy listening to. It's rather interesting (and totally logical) to heat that read has been changed to listen in the audio edition. Like when (in the forward) King (or in this case George) say that if you haven't listen to the fist parts you should do that first. In the book edition it's if you haven't read the first parts. As I said, it's totally logical but I never noticed it before now.
King himself reads the afterword to the book and as far as I understand that isn't how it's usually done. In my experience it's the narrator that reads the entire book, including foreword and/or afterword. Having it read by King himself is an extra treat. I have said it before, and I know that not everyone agrees with me, that King has a voice that works really well on tape.