I read this after reading Thud last year, which -according to the internet- is the exact opposite of how I should be reading them - the last in the series and then the first. But honestly, I'm really glad I did so.
This time around I got to read about how the protagonist, Captain Vimes, was in the beginning. In the future he becomes this stoic leader of a large Watch and the last, he's a drunk with 3 underlings. Throughout the course of the book, you see him slowly transition from one to the other.
As I said with Thud, the main things I liked were the city of Ankh-Morpork, including the characters (and seeing familiar ones introduced), the dialog, and even the whole "perspective from the bad guy" was done much better in Guards! Guards!. The humor was spot on, especially the part about the Library; his rant about Library physics is, to me, up there with Douglas Adams' diner napkin mathematics.
The only thing I didn't like was the same as Thud: the ending. I found it to be somewhat of a letdown, though I liked Guards moreso than its successor, mostly because of the difference in Vimes by the end. But in both cases, the humor, characters, and world more than made up for what was IMO a weak climax.
Overall I had a really fun time. I really love Pratchett's writing style and humor, it's just his plots that I find somewhat weak, but I consistently find myself enjoying his books whenever I have them in my hands. This one pretty well cemented that I'm going to continue in Discworld, at very least for the remainder of the Watch's story.