I have never read the book.
Because of this, I will be unable to tell you if the film does justice to the book. I can tell you the screenplay was written by Douglas Adams before he died. It is probably as close to the vision of the writer as a fan could hope.
I can also tell you that I expected the film to be much funnier.
Arthur Dent (Martin Freeman, BBC's "The Office") wakes up one morning to find bulldozers ready to demolish his small home in the English countryside. They need to make way for a new thoroughfare. His friend, Ford Prefect (Mos Def), shows up and tries to convince him that the demolition of his home is the least of his worries. Just as the Vogons, an alien race, demolish the planet Earth to make way for an intergalactic thoroughfare, Ford grabs Arthur and they hitch a ride on one of the Alien crafts. Arthur soon realizes that his friend is an alien. Ford hands Arthur "The Hitchhiker's Guide to the Galaxy", a sort of interactive road map and they set off to find a way off the alien vessel before they are caught. Soon, they meet Zaphod Beeblebrox (Sam Rockwell), the President of the Galaxy and his new girlfriend, Trilliam (Zooey Deschanel) and hitch a ride for the adventure of a lifetime.
I went to see the film with a big fan of the books. He was, apparently, quite satisfied with this film version. It has a very Monty Python-esque air about it; filled with wacky characters, a story that careens from one point to the next, creating a ride much like a pinball machine and settings that are very unusual. All of the ships and planets they visit are strange and interesting. In particular, the ship Beeblebrox and Trilliam are using, is a strange one and unlike anything depicted in film before. It has the ability to jump from one area to the next, but as it does so, it quickly changes into a series of non-sensical objects. The passengers on the ships are also changed momentarily, before everything returns to normal. At one point, everything is turned into knitted toys. When one of the passengers feels nauseous, they throw up yarn. This adds a unique, fun element to the story.
Easily, the most unusual character is Humma Kavula, Beeblebrox's opponent in the recent election for President of the Galaxy. The character is played by John Malkovich and he appears to be channeling the spirit of Christopher Lloyd. This is the type of role that Lloyd used to play, during his heyday, in the "Back to the Future" films and "Who Framed Roger Rabbit?" It is a strange character, and essentially, a quick cameo, so it is odd that Malkovich would decide to participate. Perhaps, he is a huge fan of the books as well. He doesn't add a lot to the role. Alan Rickman voices the character of Marvin, the robot. He does a fantastic job of bringing life to the depressed robot. Helen Mirren also provides a voice.
One of the more successful aspects of the film is the actual guide itself. A sort of large PDA, "The Hitchhiker's Guide to the Galaxy" is an interactive book, used by Dent and Ford, to navigate their journey. Ford gives it to Dent, presumably to help him deal with all of the weird things happening to them. Why he gives it away, instead of using it himself, is not really made clear. As they proceed, the film is interspersed with quick looks at the book. The table of contents quickly shuffles to the correct subject and we watch a brief graphic illustration of something we need to know as we continue the journey. These illustrations and the narration by Stephen Fry are very amusing and well-done. The graphics appear like something a `hip' ad agency might have created in the late 80s. It is a very unique look and keeps the story moving at a fast pace.
The beginning of the film, set in a small English town, instantly brought back memories of "Time Bandits'. It almost seemed as though the young boy in that film had grown up to be Arthur Dent in "Hitchhiker's". He has to deal with the same sort of crazy people, because he lives in a small town, and because he lives on Earth. Then he is pulled out of this world into a strange and unusual place. The beginning is very funny, as we watch Dent and Ford deal with the British locals. As the film progresses and the action moves into outer space, the laughs seem stuck on Earth. The action just doesn't translate into the type of wacky laughs the filmmakers were hoping for, you know, the type of laughs that "Time Bandits" generated. Sam Rockwell tries really hard, too hard, to get laughs. He mugs for the camera incessantly. Deschanel tries to create a character, making her seem more real than most. After getting the story going, and propelling Dent into the action, Ford, as played by Mos Def, becomes a sort of bystander, contributing little except to stand by Dent's side. As Dent, Freeman, a very funny guy, doesn't do a lot except look googly-eyed at everything going on. His character is much more interesting when he is on Earth. As soon as he becomes the center of a vortex of events in outer space, he gets lost.
There is a nice subplot involving Dent and Trilliam. He actually met Tricia (her name on Earth) at a costume party. Just as he was getting to know her, some jerk pretending to be President of the Universe, moved in and swept her off her feet. It is a nice touch and Deschanel's character has a slight edge to it. She seems ready for any adventure.
Each of the places they visit is created with a great eye to detail and they seem particularly believable. A lot of work clearly went into the special effects and the production design.
As the visual elements take over, they almost seem to overpower the actual story. During the second act, as the action moves into outer space, the story becomes more of a race. How do we get from this point to this point, as quickly as possible? What they leave behind is a discernible amount of character development and the jovial air established during the beginning of the film. This air, so essential to comedy, returns a bit during the end, making its disappearance all the more noticeable.