Well, I saw the Hitchhiker's movie last week. I grew up reading the books (I've read the entire quartet plus the short at least four times), and I found the movie watchable but somewhat disappointing in that a lot of what the characters do (in scenes taken from the book) no longer makes sense because the context in which they do it is different or unstated. This must have been a difficult problem for the producers to address, because those making the movie have to be aware of what's going to be confusing to people who aren't familiar with the book. I think they should have either taken every scene directly from the book, or made a wholly new and different story set in the Hitchhiker universe.
The producers also don't seem to realize what stuff from the books translates well to the screen and what doesn't; in particular they tried too hard to make jokes that are great in print work on screen where they just seem out of place. For instance, the dolphin scene at the beginning struck me as uninspired and silly. The Guide entries weren't the most entertaining choices available in the books, and were done in a sort of deliberately primitive animation that evokes 1980s kiddy cartoons rather than, say, graphics from a Nova or Discovery Channel special, which I think would have been a better parody.
That said, the movie does succeed pretty well in some parts, generally where it's being original as when Ford tries to communicate with what he believes is the "dominant species" on Earth. This is a joke taken from the books, but original in how the joke is applied. The special effects are also a big plus, esp the interior shots of Magrathea. And the acting passes muster; all of the characters are fun to watch, esp Zaphod.
Overall, I think Douglas Adams would be proud of the effort. Given that the movie is going to gross over $50 million, I think we'll probably see a sequel. Hopefully they'll learn from the first movie, and produce a superior second movie.