HHGTG is good. It's what originally got me started on Pratchett and Gaiman. And very few characters can rival Marvin the Paranoid Android.
Doesn't exactly hurt that Douglas used to work with Monty Python.
But Pratchett is Douglas to the Nth degree.
It's like reading Beowulf. And then reading The Hobbit.
It's not that the original is bad, it's just that people are able to build on it.
Just like the later works of Pratchett ("Interesting Times" and "Small Gods" for example), are infinitely better than his earlier works.
And whatever you do, don't watch the TV-series/Movie/Cartoon adaptation of either of their work (Or Gaimans for that matter).
Hollywood (or your national counter-part) has become very good at what it does. And it's very good at action, flash, and wonderful imagery.
But the silver screen simply cannot compete with a human imagination set alight by the written word.
You can easily have a script without a movie. But you can't have a movie without a script.
And my apologies for sounding like an 18th century librarian.