nightwatch is my (probably) favorite discworld book so far (although I must add that thud! has my favorite line in the part where Vimes is in that cave and he sees Death so Death has a near-Vimes experience and brings a book ) but I'm glad I read almost all of the others before it...it's nice to have a really good grasp of the characters and then find out what they were like when they were younger...all throughout the books the characters are really developing (Vimes turns from a drunk to a married duke with a kid etc.) and then T.P. gives us a book that takes us to the begining...I liked the idea of Vetinari as a young assasin learning himself to be invisible...it just fits him so perfectly...

p.s.
I'm a discworld fanatic too or a Terry Pratchett one when I think about it, but there are four books I didn't like as much: The Colour of Magic & The Light Fantasic (because I had the feeling that T.P. didn't really have a clear image of characters yet and also that his writing style wasn't as polished as it is today...I would probably like them better if I started the series with them though), Last Continent (it just leaves me puzzled about so many things...I just didn't know what was going on through half of the book the first time I read it) and Monstrous Regiment (it's really improbably to me that everybody would be a woman faking to be a man...)
Although I must say I loved those four books a lot more than 99% of any other book written by any other author...in my opinion the only other two authors who can be compared to T.P. are Douglas Adams and George R.R. Martin...at least the modern ones...Tolkien has the same quality too...if Terry Pratchett (or the other four writers) wrote a guide about how to fill out your incredibly boring tax papers it would be still be written so well that you would just be dragged into it...the writing is just harmonious no matter what they write...