Mostly Harmless.
There. That's it then. Not horrible. Not exactly Douglas Adams. Also not not entirely unlike Douglas Adams. It was funny if you forgot the funny line was out of context or incomplete (which was very common)-- just not uproariously so. Not replete with slapstick, but then again more American with its slapstick than, say, Python. Far less verbal a comedy than Python, let alone Douglas Adams's work.

Two and a half cups of nice, hot tea out of a possible five.
Rather than bitch about its lack of faithful adherence, I'll just mention some of what I liked:
Mos Def was an appropriately distracted alien. A Ford Prefect not quite attached to earthling concerns, a little odd. Quite likable. Sam Rockwell's Zaphod Beeblebrox was funny as an somewhat air-headed figure head, and I'm sure the George Bush accent helped that. But well done. In fact, I think that most aliens sounded American may have meant something here. Martin Freeman's Arthur Dent was a little more sympathetic, but not particularly stupid, always saying the obvious and freaking at he was originally written. But he is the hero of sorts and so it works. Trillian was, well, almost ok. The love interest detracted from her character, and I think without it Zoey Deschanel could have made the role shine.
Bill Nighy's Slartibartfast was appropriately off and vaguely distracted enough to give the impression of being an alien. Still, more clueless "What?"s from Arthur would have thrown the character into sharper relief.
The Vogons, though not very loud and nasty, were big beauracratic slugs and that came off ok. Clownish but a glimmer of the brutish and overwhelming horror that is the Universe, a strong theme in the books.
Now for a quick bitch: where was the abject existential horror in Marvin's demeanor? Why did the forget all about the Guide halfway through? They under-utilized their two great vocal talents-- Alan Rickman and Stephen Fry-- and by doing so managed to take the juice out of those parts as Adams wrote it.
It wasn't a completely loud, pow-bang-whoosh movie, but it came periously close. Its underplaying of Adams' finely crafted dialogue really lessened the film. I think it would have made it much stronger even plotted out as it was if they had included ALL of the sometimes quoted, sometimes paraphrased or even misquoted classic Adams lines. A less verbal film than the radio show or TV series by half and that really was to its detriment.
So 5 out of 10 and it could have been worse. Not completely awful. Mostly harmless.
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