lordandrei has a good non-spoilery review here, so I'll go straight to a slightly more spoilery version.
Overall, it was good. Quite good! They Hollywood-ized it a bit, only to be expected - the biggest change from all the other versions, really, is that they inserted an Arthur-and-Trillian love story, and sure enough he gets the girl at the end.
I know that alone is enough to make a whole lot of you groan in despair - but, honestly, it wasn't that bad. I thought the development of the love story was a little uneven, and the ending was way too neat and clean; but it wasn't overdone (if it was going to be done at all), and it managed to give both of them some surprisingly believable character development.
The characterizations were, on the whole, magnificent. Arthur (Martin Freeman) had the whole bewildered 'never could get the hang of Thursdays' air without being as flat and stupid as he came off in the radio/tv series. Trillian (Zooey Deschanel) was a bit toned down, more 'normal' than in previous versions, but she had just enough of the wild-eyed air to make it quite believable that she'd run off with Zaphod.
Ford, played by Mos Def, was fucking brilliant. Came off as much more an alien than in previous versions, and had many of the old familiar lines with a new, but just as funny, twist.
Zaphod - ah, Zaphod (Sam Rockwell). You'll either love him or hate him (I loved him, Andrei hated him). It's, well, a very strong characterization. Much more looney, less 'cool', than the tv/radio version. And yes... he does have two heads. But not in the way you'd expect.
And then there was Marvin, voiced by the immortal Alan Rickman. Visually he is quite a shock (you can see him in the center of the toys poster here). I wasn't blown away by him - but he was good.
I was very impressed by Slartibartfast (played by Bill Nighy. Stephen Fry as the voice of the Book was also perfect, very like Peter Jones in the original.
There are a few of the lovely interludes with entries from the book, complete with amusing computer animations. Oolon Colluphid's trilogy, the Ravenous Bugblatter Beast of Traal, the coming of the Great Hankerchief, the bowl of petunias and very surprised sperm whale - all present and accounted for! (No digital watches, though. Alas.)
The very last shot of the movie, as they jet away to visit the Restaurant at the End of the Universe, is the ship flashing through its usual set of improbable objects, ending... with Douglas Adams' face.
Of course, this was the rough director's cut. It will be interesting to see what they change for the released version (which I will definitely pay to go see). But I really don't see them absolutely ruining it at this point. There's too much there that's good. And they've left the door wide open to do the rest of the series, though I'm afraid this won't be mainstream enough to get picked up for the sequels. We can hope, though!
There's too much to go through play-by-play, but if there's anything in particular you want to know, comment and ask, I'll answer :)