I saw the film last night, and I am as rabid a fan as Skar is. I think his review is almost right on the money, except that I liked Marvin and was disapointed in Rockwell as Zaphod (the Elvis style voice didn't work for me, and his best lines were all gone.). Freeman was perfect, and I LOVED MOS DEF. I freaked out when I heard they had cast him the part, but he was fabulous. Deschanel was a big step up from the awful Trillian in the tv series. The weakest link in the cast is John Malkovich, which can be said of almost any movie that features John Malkovich. I agree about the "Guide" entries - they were the best parts, and the casting of Stephen Fry as the voice was sublime, but I could have used more entries.
All in all, I would compare more to "The Bourne Identity" than "LOTR." I make this comparison because, as a Luldum fan, I hated Bourne when I first saw it, because they threw so much of the book out the window, but upon repeat viewing, particulary in relation to "Supremacy," I have grown to like it a lot on it's own merits, and feel that it is in the spirit of Ludlum's work. "The Guide" should be viewd the same way- it is not definitive, but Adams never wanted it that way, and each new version can bring us something new.