I was all prepared to be a hard marker with this one - I didn't see how the books could translate effectively to film, I'd seen the filmic Marvin and thought he was way too cute, and hey, they're messing with literature for which I've had some serious love in the past (and which, for better or for worse, has definitely left its mark on me). But the initial sequence with the dolphins went a long way to winning me over, and I thoroughly enjoyed the rest of the film - it wasn't uniformly funny, but there were no 'down' or dull parts for me, either.
Oddly given my expectations (but perhaps not so oddly in another sense), I think that this was a film best appreciated by those who are familiar with the books (and, presumably the radio/tv series, neither of which I know much about) which comprised its source material - that kind of familiarity gives the film a richness and a sense of unifiedness that I suspect would otherwise have been lacking. This works in a global sense (grasping what it's all about), from scene to scene (anticipating the payoffs with jokes and planted clues, such as the two mice, say, and enjoying the added scenes like the strangely hilarious idea-discouraging spades buried in the sand of Vogsphere...the subliminal joke there being that the Vogons, as bureaucrats, of course never have any original ideas...of which there are a surprising, and effective, number), and on the level of minutiae (David pointed out to me afterwards that the original tv-series Marvin actually got a cameo in the film).
I thought (again somewhat against expectations) that everyone looked right - Arthur and Trillian certainly, Mos Def as Ford Prefect made sense immediately (though I'd always visualised him as a Harrison Ford/Han Solo type, for regrettably obvious reasons), Zaphod was good once I got over the disappointment of where his second head was (ingenious enough but I would've liked them side by side on his neck), Slartibartfast was perfect, and as to Marvin, well, it wasn't as I'd always imagined him but I didn't find the change too offensive and it worked in its own way. Overall, I felt that the Adams feel was preserved quite well - the special effects were both impressive and somehow dinky, and the Python-esque feel of the whole was appropriate enough (and let's not forget the improbability drive-induced craziness which was actually shown). So, no complaints in these parts - this is jolly good.