Hogfather is one of my favorite DW novels, for all the reasons others have been talking about.
At the same time, I find it one of the hardest books to re-read. There's just so much going on in it and the plot and ideas are so complex that it's not easy reading, and I can see why people would be put off by it.
Teatime is Pterry's best villain, even better than Pin and Tulip. Had he channelled his genius and sociopathy into politics instead of crime he could have become the Patrician.
While I thought the movie version of Hogfather was the best of all the DW adaptations to that time, and it was nearly totally faithful to the book, I found it incredibly slow moving and lacking in energy. The role of Ridcully in particular was woefully miscast; he was made to look and sound like a weak and doddering old fool, instead of the larger than life, robust, garrulous, outdoorsy man he is in the book.