[I read Thud some time back, but recently came across this good-condition hardcover at the annual yard sale to benefit the Humane Society of Greater Nashua, and couldn't resist picking it up as another release copy.]
This is another in the "City Watch" sub-series, which was my first introduction to the Discworld and remains one of my favorite things about life on the Disc.
While I enjoyed the book, I wouldn't put it in my top-ten-favorite-Discworld-books list; it seems a bit more somber throughout than most of the series, and while there was still plenty of humor, it didn't zing the way I'd hoped. [I don't think it's a case of more-recent-book vs. older books; I found Going Postal to be one of the most entertaining Discworld books so far.]
That said, I still found Thud! quite entertaining, from its riffs on games and gaming as a way of getting to know different cultures to the scenes involving Nobby's latest girlfriend (a pole dancer at the Pink Pussycat Club), City Watch werewolf Angua, and the Watch's latest acquisition, the vampire Sally. [There's a bit with Angua and Sally and a mud pit beneath the city - well, you had to be there, but trust me, the quip is priceless!]
I liked the vaguely Lovecraftian feel of the various "Dark" runes that start to crop up everywhere - a very nice touch. [If I inflict the Summoning Dark or the Following Dark on the players in my next D&D game it'll be Pratchett's fault.] And I was charmed and amused by Sam Vimes' new air of domesticity as he insists on being at home every night at 6 to read young Sam his favorite story [which has also been released as its own book, sort of - see Where's My Cow?]. [The fact that this actually entered into the plot later on impressed the heck out of me.]
Other bits that I liked: after a series full of dwarves named Stronginthearm or Helmclever, I was tickled to meet one Bashfull Bashfullsson!
And: fairly early on in the story there's a brief but memorable scene involving Sybil and her swamp dragons that's worth the price of admission all by itself.
And: while I've managed to avoid most of the Da Vinci Code hype [nothing I've heard about the book itself has interested me much, and I have a skeptical attitude about people finding codes in lengthy tomes - humans are a pattern-matching species and can often find what they look for whether it's there or not], I had gleaned enough about it to recognize it being parodied in the Koom Valley Codex subplot.
Conclusion: even a not-in-the-top-ten Discworld book can be a lot of fun!
[There's a TV Tropes page for the book that's entertaining, but do beware of spoilers.]