I just read the books recently, and I have to say the movie seemed to be faithful to the books. Alan Rickman was great as the voice of Marvin. I think that the narrative was crucial to understanding what was happening, and a lot of the humor in the books was transferred in the narrative and the animated HHGTTG parts - I loved those. I liked it a lot, realizing that a book != screenplay and some changes have to be made to make it work on the big screen.