This site (
http://planetmagrathea.com/shortreview.html) has thee articles by a guy who saw an early press screening of the film, and it is with heavy heart that I tell you he absolutely hated it. Fortunately (or unfortunately) he gives more than just his opinion: he gives a short review explaining the overarching problems, a brief look at all teh stuff not included in the movie, and a long, four-page spoilerific summary opf the entire show.
If you don't want to read it all, let me sum it up for you: the movie makes passing mention of some events in the book, but takes them all out of context and invents several new scenes that have no point, drama, or resemblance to the story as we know it. Worst of all, the dialogue (which is really the whole point, given that even Adams' original story was episodic and underdeveloped) has all been changed. Adams jokes all are gone, replaced by new, lame jokes that could have come from any bad Hollywood comedy. Zaphod's character, background, and motivation are completely changed, Marvin (arguably the most popular character, and certianly the most quotable) has virtually no lines, and the Guide itself is all but absent (and its narration, when it actually shows up, is completely different).
On the good side, he said that Slartibartfast is quite good, and Trillian is very well acted and, ironically, the most interesting and compelling of the main characters.
Obviously the guy who wrote the reviews is an obsessive Adams fan, so many of opinions are a little over-the-top. Nevertheless, if even half of what he says is true, this movie is a wretched, unfunny piece of tripe that is so far removed from the Hitchhiker's Guide we know and love as to be unrecognizable.