You should've seen it~
Excellently cast: Adora, Groat, Stanley, Angua. Poorly cast (in my opinion anyway): Reacher Gilt, Drumknott. Moist I was dubious about, but I think it was the script which didn't do the character justice rather than the casting. Vetinari worked very well in this specific context, but I think I'd want to see someone else in the role if they did, say, Jingo.
For the most part the changes to the storyline worked for me. I particularly liked bringing Angua in as the arresting officer, and Adora getting a scene with Mr Pony. And the 'divine intervention' scene was done very well. Wasn't a fan of the changes to the causes-of-death for the former postmasters. My biggest single gripe was the changes to characterisation in Moist and Gilt. They're both meant to be world-class con-artists, and it seemed to me that in the film they both worked messily and lost control a little too easily.
One last fairly random thought: was rereading Going Postal, and it struck me that book-Moist reminds me a lot of Crowley from Good Omens.