The HHG2G movie frustrates me a lot. All the elements are there: a script (mostly) by Adams, decent special effects, a creative team that really did appear to love the matieral, and Alan fuck*ng Rickman as Marvin. But the final product just feels so...empty to me.
I think a big part of it is that Adam's prose just loses so much when people say it outloud. Well, that's not true. It needs to be dry and British, and this movie was mostly American. All the good lines were there. Most of the best guide entries. But they just kind of fell flat as presented in the film.
Another good example is the Whale scene. Funny in the book, just kind of weird in the film.
And Marvin...I love the depressed iPod design, Alan Rickman is who I wanted to play him the second I heard about the movie...but he's just so depressing in the movie. Not like funny depressing, but actual depressing. It's a thin line, but they ended up on the wrong side. His "I've seen it...it's rubbish" line about the Magrathean sunset was one of the first times I've laughed out loud reading a book, but in the movie I barely even noticed it.
Here is what the movie does right: The dolphin song,the whole part on the Vogon homeworld, Slartibartfast (and really most of the stuff on Magrathea), and I actually though the movie's ending was pretty decent. It could have been a lot worse. I think the people who made it really did love and respect the material, but they tried too hard to stick to it (when not deviating from it altogether). I actually liked the added scenes more than the stuff in the book because the added scenes were written for the screen, and seemed to work better. I would have preferred a less faithful movie that was funnier in movie form, if that makes any sense?