Watched as much of the 2005 film as I could stand (up through the scene where all the main characters meet on the Heart of Gold), and so, here's a review.
The facetious version: Watching this movie unfold is like being a pregnant woman, going into labor and being wheeled into the delivery room, fully aware of the magnificent potential of the nascent and beautiful life to come...only to have the nurse say, "Your regular doctor is unavailable, so we've brought in Leatherface to deliver your baby. He knows a lot about anatomy, right?"
The lengthier, more accurate version: It's like watching the real Hitch-hiker's Guide movie, as re-enacted the next morning by the guys around the office who were pretty drunk when they saw it, aren't necessarily sober now, and don't remember it really well. The film tries to "improve" on the original (in all its various forms) by botching punchlines, snipping those long talky bits in favor of a romance between Arthur and Trillian, taking great descriptions and turning them into unfunny sight gags, and then having the whole thing be performed by a cast of actors who wouldn't be able to make the cut in a high school production of 'Our Town'.
Martin Freeman completely misdelivers one of the best lines in the story ("We've met") so thoroughly that you have to believe it was on purpose as a means of showing his contempt for the other actors, Mos Def delivers each line as though he expects the director, at any moment, to shout "ACTION!", and I'm firmly convinced that Sam Rockwell is trying to kill me, using his acting as the murder weapon so that the police won't suspect him. His delivery of the line, "Hey baby, is this guy boring you? Why don't you come and talk to me, I'm from another planet," is eye-bleedingly, tooth-grindingly, hideously, Shatnerianly bad, and he just gets worse from there. Zooey Deschanel is decent enough as Trillian, but her character has suffered the most from being rewritten, to the point where we're actually supposed to believe that a smart, clever, witty woman usually asks guys she's just met 15 minutes ago at a party to quit their job, sell their home, and travel to Madagascar with them...and thinks the guy's boring when he doesn't do it. Yeah, something tells me that's a recipe for perpetual disappointment.
Oh, and Marvin looks like a stormtrooper who's just swallowed a weather balloon.
I'm still of the opinion that a good Hitch-hiker's movie can be made. But this is as far from it as you can possibly get short of having it be a one-man show starring Carrot Top.