Thief of Time is simply superb - just finished it this week.
It's an incredibly wonderful mix of observation, philosophy, and comedy of the highest degree.
I had read Going Postal before then, which is quick and fast, and opening Thief of Time I thought the first few pages felt lacking by comparison.
However, soon the story gears up and Pratchett just hits everything hard and fast - his philosophical commentaries are hilarious, and his treatment of the entire story, concept, and execution was incredibly polished.
I couldn't imagine any other writer being able to achieve as much as Thief of Time without spending years on it to get it to such a high standard of polish - but by all accounts, he can roll them out.
There's nothing tired or forced about this book - Terry is sharpest than I've ever seen him, and whole experience was very fulfilling.
If this were made into a film, it could only become both a classic fantasy and classic comedy at the same time. I mean, really, the Four Horsemen, Ronnie, the History Monks, Death, Susan, the Auditors, and chocolate?
ADDED: Changed the thread title for better attention on the topic.