I rented "The Hitchhiker's Guide To The Galaxy" on DVD and watched it last night.  What a funny, charming film!  I read the book when I was teenager and even got a chance to see the BBC produced mini-series (I was a "PBS Kid" back before it was cool to be one).  While I'm not as big a fan as most, I believe that to enjoy the film you had to have read the book or at least a strong familiarity with the satire in the story.  Douglas Adams's literary work permeates much of our pop-culture still, so if you're paying attention you'll get a chuckle out of it here and there. 
I give it 3 out of 4 stars.  I would give it a 4 but I'm sure if I were bigger fan I probably would probably nix it to 3 anyway.  ~pg
 
