I wasn't a huge fan of Rickman as Marvin...I'm not sure what it was but ti didn't sound right to me...but in saying that, I've grown used to and loved the radio series and preferred the Marvin voice on radio. I still liked him though.
I really liked the film version. I thought it was very different to the book and radio series, but was a very good version of the 'story.' IMO, the h2g2 story suits the film medium perfectly and you could see this with all the sets and landscapes which were simply beautiful. A lot of shit was thrown around on the imdb boards about Hammer&Tongs being first-time directors, but I thought that their vision was gorgeous and really made the film. I loved Sam Rockwell as Zaphod...while it wasn't the same Zaphod I knew, it was still a very funny character (I absolutely cacked when he was dancing around on Viltvodle while getting shot at *peoo peoo peoo peoo!*)
Mos Def did an awesome job as Ford, the highlight being when he attempted to shake hands with a car assuming it was Earth's dominant life form. Martin Smith was a capable Arthur Dent...and Zooey Deschanel was gorgeous...I never realised how hot she was, but she really lit up the screen in all her scenes. Bill Nighy has been one of my favourite actors since Shaun of the Dead and Love Actually and I don't think anybody could have done a job as awesome as he for Slartibartfast.
LOVED the movie...and I'm not fussed at all about the changes made from the book/radio series...it's an excellent version of h2g2 and I'll be more than happy to see it again.