First off, let's get one thing out the way: I am a fan of the books, most definitely. I've read the first four, and also the Dirk Gently Omnibus. Douglas Adams was a genius, God rest his soul.
I wasn't entirely sure what to expect from this film though. I've skim-read reviews, but I've tried to resist the lure of spoilers, even though I wasn't sure if I'd go see it or not. Well, now I did.
The major problem with this film is that, even though the book is nowhere near as thick as, say, any of the Lord Of The Rings books, Douglas managed to fit in a hell of a lot of detail. A proper film version would be nearer LOTR-length, but I'm aware that whilst a trilogy of fantasy films of epic length may not be too difficult to sell especially given the success of the LOTR books, if you were to say "I'm gonna make a trilogy of epic comedy films" I'm pretty sure you'd get laughed all the way back to whatever bar you'd just stumbled out of.
Given the restriction in length, they have managed to pack in the vital-to-the-storyline parts. I think - it's been a little while since I read the books, I'll have to read them again at some point - and whilst there are plenty of "incidental" little bits of humour that have been dropped, one or two have also been added.
The dialogue is somewhat cut down. I think the Vogons and the bit with the custom planets are probably the most well-done parts of this film, they have the best "feel", they're most reminiscent of the book. I was somewhat disappointed with the rendition of the Heart Of Gold, I think they were trying to be a little too clever there and...it doesn't quite work. I have to admit, one reason I like the film's vision of the Vogons is because they're a little reminiscent of the aliens from Peter Jackson's masterpiece of comedy-horror-gorefest, Bad Taste.
I was surprised at how often much of the audience actually laughed out loud, but I quickly realised most of them have probably not read the books, in which case, you're likely to enjoy this film rather more.
I just hope the next one is better. I can see that the potential is there, it just seems almost as if...they were aiming lower than the books to begin with. As if it went something like "We won't make it as good as the books so let's not even try to, just take the easiest and most important bits and work on them"