I love Going Postal, I'm one of the it's-in-my-top-ten camp. Even top five.
Re. the debate about how people felt about Moist's character - there's a difference between liking and enjoying a character. I thoroughly enjoy the character (in GP, not MM which I broadly disliked) and am very happy to spend this book following his story and hearing his thinking.
It doesn't mean I'm blind to that fact that he begins, and remains, essentially a bastard. Circumstance (via Vetinari) push him into a situation where his considerable skills can be used to benefit the city, and consequently Moist begins to change his attitude a bit. But his basic personality is still deeply flawed. Lord V sets a thief to catch a thief (or sets a spring-loaded pit etc etc....) by putting Moist on the case. Only someone with a grifter's nature could take on Gilt. Moist is the man for the job, and the job benefits everyone. But that doesn't make Moist a nice person. It's Moist's awareness of this that makes him sympathetic to me.
And I enjoy watching him be clever, for the same reason I enjoy a good heist or spy movie. It's fun to watch him outwit everyone.
Terry was also clever in the dynamic with the villain. There's books where it's good versus evil and the main character opposes everything that the villain stands for. But I've always enjoyed a story where the villain represents the worse side of the hero's nature. Someone the hero is able to look at and define themselves by not being. I think that's the major prompt to righteousness for Moist in GP, more that Lord V's machinations or wanting to convince Adora or anything. It's wanting to not be Reacher Gilt. The film kind of missed this point and didn't draw any comparisons between Moist and Reacher, I think to its detrement. For Moist to be heroic, he has to be confronting the version of himself that is villainous and working out what makes him different from that person.