In short, excellent. I think they probably could have done better with Reacher Gilt, but only because he didn't quite work in a couple of scenes. The rest of the time he was fine. Charles Dance as Vetinari and Ingrid Bolsø Berdal as Angua were particularly great, but really they were all pretty well chosen.
Richard Coyle as Moist was far better than I'd anticipated. Having only seen him before in Coupling I was expecting to have difficulty believing him in the role, but it really wasn't an issue at all. Tamsin Greig as Sacharissa was another one I thought I might have trouble with, because when I saw Shaun of the Dead I couldn't help but see her as Fran from Black Books, and she also played a fairly Fran-like character in Green Wing, but she was really good in this.
While neither as funny or as deep as the book, part one really has very little going against it. The changes made were, by and large, necessary to make the transition to TV, and it worked really well, with one exception. The blatant anti-smoking propaganda shoved right in the middle of the show for no reason. I'm not saying that TV shows should encourage smoking, but really, do you have to be that obvious about it? The episode practically stopped to say "Hey kids, in case you're wondering, smoking is bad and it is a very bad thing that Adora smokes!" It was really jarring and felt completely out of place.
Other than that though, it moved along at the right pace, it left in enough to be the same story without trying to keep every little thing regardless of how badly it would work on TV or how long it would make the show, and all in all, it was about the best that could be expected.
Part Two, unfortunately, faltered a bit. Don't get me wrong, it was still good, it just wasn't as good as it could have been. I think it started to move a little bit slower here, and dragged on a bit, until it reached the end at which point things just sort of reached a conclusion without really telling you how it got there. There seemed to be a lot of loose plot threads still hanging around and everything just sort of suddenly wraps up in an unsatisfactory way.
Also, an attempt was made in this part to make Reacher Gilt seem genuinely threatening, and it just didn't work for me at all. He was far too much of a cartoon villain to take seriously. A psychopath threatening to drop a girl off a tower should be quite a tense scene, but it just wasn't. They got this right in Hogfather with Mr Teatime, but apparently they were not able to duplicate that here. Even the scene of him beating a man to death and dragging the body away wrapped in a carpet just came off as very unreal and made no real impact.
Like I said, it wasn't terrible, and I still enjoyed it, it just left me a little unsatisfied and I felt that it could have been a lot better.
Then we come to what i feel is the weakest part of the entire thing. After we saw that Moist has won and everyone's happy, we get a couple of extra little bits.
First we have Reacher Gilt being arrested by the Patrician in the same way Moist was at the start, and if the real ending felt rushed, this felt like it was tacked on as an afterthought. And why the postman? Couldn't you have chucked Pratchett's cameo in somewhere else? It made no sense for him to be there. How did the post office know to deliver to Gilt in the palace? Who knew he was there but a few trustworthy watchmen and some high-ranking palace staff? Also, I was quite disappointed that they left out the bit about Reacher absolutely believing in freedom of choice, which I felt was probably one of the best bits of the book.
Then we have the bit with Groat and Stanley losing their winnings. I thought for certain they would handle that better, because the idea that Moist couldn't hurt anyone with his plan except Gilt was pretty important, especially with all the bits in part one where he sees the error of his ways after learning of the harm he's caused, but here the issue is sidestepped and Groat and Stanley end up losing their money, and an important part of the story is lost to a really weak joke.
I understand the desire to make Gilt a cartoonish villain, and I think it would have been fine, if they could have stuck to it the whole way through, but at various points it becomes necessary for him to pose a genuine threat. It was fine when he was a threat only by proxy, sending Mr Gryle to do his dirty work, because that fits the cackling cartoon villain mould very well, but when he has to get his own hands dirty, it just doesn't work. I feel it would have worked a lot better if they could have decided once and for all whether Gilt was supposed to be scary or just a caricature of scary.
I really did enjoy quite a few of the Patrician's lines though, and Angua was pretty cool. I probably won't watch it again, but it was worth seeing once.