Simply stated, this is one of Pratchett's best. He brilliantly skewers politics, warmongering, racism, engineering, time management, navigational terms, police work, and foreign customs, for a start. He then manages to poke fun at (as opposed to ridiculing) humans, werewolves, dwarves, trolls, gnomes, Curious Squid, and the odd orangutang. His treatment of the temperature-sensitive intelligence of Corporal Detritus is well done, and the habit he gives of Commander Vimes returning to his old habits as a street copper even in the middle of ceremonial events had me rolling on the floor.
The demonic organizer and the temporal slip-up was a very nice touch... although the reeling off of the appointments in the alternative final defense (which I shall skip for those who haven't read this yet - I envy you people! <grin>) I found honestly to be as chilling as anything Stephen King ever wrote.
I don't believe I've ever read a bad book of his. Some may have been better than others, but not one of them have I put down and said, "My God, why did I read this?" Pratchett writes with a compassionate eye to his characters, keeping them comical without making them ridiculous.
These are books that I'll still be re-reading 20 years from now, and I'll wager I'll still find something new to laugh at each time.