Just came back from seeing the film version, and wow! A lot of people (most notably Planet Magrethea) have criticised the film as not being true to the books/radio/TV and quite frankly, bull. Who cares, every version has been different and whilst the additions aren't brilliant, it's still a fantastic film.
The way the Guide itself is introduced is great - using Journey of the Sorcerer, the Hitchhikers theme - making the Book the 'hero' of the film. Martin Freeman fits well into the role of everyman Arthur Dent (although at first the performance will probably jar with most Hitchhikers' fans) and Sam Rockwell was inspired casting. Mos Def and Zooey Deschanel both turn in solid performances too and Stephen Fry was another excellent casting choice.
The fact that this is Garth Jennings directorial debut does show through in parts, you get the feeling he isn't showing everything on screen that from a technical point of view he should, however he's experience directing music videos also adds a dimension to the film that probably wouldn't have been there if a more 'mainstream' director had been chosen.
Go out, see it, but don't expect the book/radio/TV version. Don't Panic.