I read this book ages ago and thoroughly enjoyed it, this TV adaptation by Amazon has been just as enjoyable. I was really reassured when I saw Neil Gaiman was involved in writing this script. It more or less holds very well to the original book. The pacing feels good too, not too rushed, but it doesn't drag on either.

The casting is SPOT ON! I will admit I'm a fan of David Tennant anyway, but his portrayal of Crowley is just fantastic. All of them really fit their roles to a T.
I also appreciate the use of a narrator, I think that was a great stylistic choice. It not only helps it hold to the book more closely but is also a good way to add a little exposition where some might be needed without it feeling forced or awkward/clunky.

And of course there is plenty of the classic Terry Pratchett wit and humor to be found through out. It's quirky, but not so goodfy that it fails to take itself seriously when the situation calls for it. It's one of those shows I wish there could be more of, but all good things must come to an end, and of course being based on an existing book assures that won't happen either. ^^;