I realize that the blogosphere will be brimming this morning with reviews of The Hitchhiker's Guide to the Galaxy, but I'm going to share my thoughts anyway. I'll do my best not to give away anything important, but if you haven't seen it yet and want to go in fresh your time may better be spent looking at pr0n or something.
I saw it yesterday with Meli and four of my geekier law school friends, and it got a universally positive reaction from all six of us. The key thing to note is that, yes, the story gets changed around quite a bit -- there's a whole new plot line thrown in, and the ending is significantly different -- but supposedly every new element originated with Douglas Adams, so that lends some legitimacy to the whole affair. Also, many of the changes were necessary for the purposes of making the book into a watchable movie. Finally, the changes are all pretty cool, particularly the new plot line. The creators have made it clear that the film is simply one more "version" of H2G2, so just as the book wasn't a direct adaptation of the radio series, the movie isn't a direct adaptation of the book.
The casting couldn't have been better. I know people are antsy about Mos Def playing Ford, but it really works. He comes off as much more "alien" than he did in the books or the radio series, where he was just another Englishman who happened to be from another planet. Sam Rockwell was fabulous as Zaphod. The two-head gimmick (having his second head hiding in his shirt for the most part and representing the more aggressive parts of his personality) worked very well, and effectively avoided the awkwardness of having a single character with two heads on his shoulders. The actress playing Trillian was good, though in some shots vaguely resembles the hated and not-hot Reese Witherspoon. Marvin, bodied by Warwick Davis and voiced by Alan Rickman, was well-portrayed, even though he was too short. And Arthur Dent, the hapless, overwhelmed everyman, was perfect.
One thing I noticed is that the movie doesn't lampoon philosophy nearly as much as the books. This is a little distracting, since it was such a major driving force behind the books. The meaning of life stuff is still there, obviously, but a lot of the direct attacks on philosophy [SPOILER!!!!] such as the Majikthise and Vroomfondel sequence [/SPOILER] are conspicuously absent. Admittedly, it's hard to make fun of philosophy in a movie, but that was one thing I noticed.
A final point is that, if you haven't read the books, you'll have a tough time with the movie. A lot is left out and glossed over. Scenes and concepts are heavily condensed. If you didn't develop an encyclopedic knowledge of the entire trilogy in junior high and/or high school like most Hitchhiker's fans, you may have trouble filling in the blanks. But it's still a fun film to watch (if only for the completely ridiculous opening sequence), and if you see it with one of your geeky friends you can go out for coffee afterwards and have him/her explain everything you missed.