I find them a bit lacking, compared to his later stuff. They are deliberate parodies of the fantasy genre, and the joke kind of wears thin. I am almost alone among my friends (I'm British, so I have lots of friends who have read Pratchett) in thinking that his stuff just keeps on getting better (most of my aforementioned Pratchett-reading freinds have discworld fatigue. He is a wee bit prolific). His latest stuff has better characterisation, it's darker and it has more to say about society and less about obscure pulp fantasy books I have never and will never read.
I'm with you on that. The Discworld series started as pure parody and nothing else -- and I personally found the first two books very funny as a teenager. (The first one in particular; I still remember the scene where Twoflower tries to pay his tab at the seedy tavern with pure gold.) I haven't tried to read them since.
His parody gradually spread out, picking more diverse targets. He had a lot of one-gag books like
Moving Pictures (the fantasy world invents Hollywood) and
Soul Music ("sex, drugs, and music with rocks in.") Even at their shallowest they were still pretty funny, or at least funnier than a lot of the other stuff out there, but they didn't stick with me.
Then I read
Small Gods. That book
stuck. It's the first time I felt that his satire was trying to say something really interesting -- in this case about religion, and how gods need believers more than believers need gods. On a side note, it's also the first Discworld book I pushed on Anna that she liked.
Since then he's been all over the map. Some books are still silly satire, and there are some that just plain aren't good (
Jingo and
Thief of Time did nothing for me), but sometimes he gets deep.
Really deep. And the jokes are always there, but sometimes the jokes are serious. I'd put
Night Watch up against just about any other fantasy novel for complexity, character depth, and capacity to astonish. And
Wee Free Men is one of the best YA novels I'd read in a very long time.
So yeah, the series has definitely gotten better with age. Not with total consistency -- like wine or whisky, some barrels just don't come out right -- but on the whole.
And if you're just starting Discworld now, and you intend to read it through, man, are you going to stay busy for a while. >8->