Finally got around to watching, having picked up the DVD: The Colour of Magic.
This is one of those movies - often fantasy - which primarily exists as an adaptation of a popular book. People who haven't read the source can sit there trying to work out what's going on and what the point might be; people who have are the primary target market, and can sit there being impressed by the realisation of the book's imagery. Not having read any of the Harry Potter books, I know how it feels to be in the first category; having read all the Discworld books (I can even say "twice" for some of them, sort of, which is unusual for me - but that was for professional reasons), I could really rather enjoy this. Ankh-Morpork looked the part, if not quite big enough, and a lot of other details were dead on.
The script was a pretty fair adaptation and condensation of The Colour of Magic and The Light Fantastic, only very occasionally feeling rushed, mostly dropping the right things, and getting one half-decent joke (all the Bel-Shamharoth references) out of the excisions. One small necessary addition was the explanation of why Rincewind should be David Jason's age, but that doesn't particularly explain why David Jason was cast as Rincewind in the first place. (To sell the thing outside the guaranteed target market, of course.) Rincewind should have been played by a younger Nigel Planer - but instead we got the current Nigel Planer not doing very much as the Arch-Astronomer of Krull. Hey ho.
And some of the casting was fine. We even got Jeremy Irons in a cameo as the Patrician, which is as good as it gets, at least in the absence of Alan Rickman (and yes, I know that the Patrician of Colour of Magic the book isn't Vetinari - let's apply a no-geekery rule, shall we?), and Tim Curry had and was enormous fun as Trymon. (Rule #7 of cult movie making; let Tim Curry enjoy himself, okay?) Karen David and and Liz May Brice got Liessa and Herrena right, and David Bradley managed Cohen the Barbarian dead on, at least in the talkie bits.
But that reminds me of the other notable problem; the action sequences were a bit feeble. Pratchett may be a comedy writer, but he's a comedy writer who can handle action immaculately (the climax of Moving Pictures is a masterpiece of choreography), and he's working within and around the traditions of pulp action fantasy. Cohen ought to be Conan with creaky joints, a deadly-subtle combat style, and a screeching battle-cry; the interior of the Wyrmberg ought to be up there with the Mines of Moria for gob-smacking awe and high fantasy violence, subverted only by Rincewind's terror. This treatment just didn't cut it in those moments; they were just sitcom punch-ups in funny costumes. (The Wyrmberg generally was squeezed in and thrown away.) And while the special effects generally weren't bad, they showed worrying signs of budgetary tightness; the giant troll looked okay, but rampaged against the skyline rather than smashing up the scenery, while the brief "characters on horseback" shots were pure 1960s back-projection.
But, you know, whatever. The fans will catch this, one way or another, and the non-fans who find themselves trapped will have David Jason to watch or some good incidental Pratchett gags to listen to, according to taste. It could have been more - it's perfectly possible to adapt literary comedy to good effect (see the better past Jeeves/Wooster TV projects, for a start), and a bigger budget for FX and fight arrangements would probably have made a big difference - but if we're going to have Discworld movies, getting some bits right is a start.