My family had been reading Pratchett for years, but after not being able to finish The Colour of Magic when I was eleven or twelve, I had assumed he wasn't for me. I picked up Thud in the library soon after it came out, and, not having a library card, I sat in a comfy chair and read until closing time. I came back early the next day and finished it. It is probably one of the best books out there and, because it introduced me to Pratchett, is one of my favourites.
Like all Discworld novels, it's a stand-alone book. The only problem I had with reading it first was that it's got a few spoilers for earlier books (ie: Cheery. That's all I'm saying).
I would recommend this book to anyone who loves fantasy, hates fantasy and/or is a fan of clever prose.