H2G2 is kinda a really weird bird. The original radio show was it's own thing. Then they recorded the scripted radio show for LP- and they changed some of the story on that version. Then Douglas wrote the first 2 books- and expanded things and changed things that were different from the Radio shows AND the LPs. Then they adapted the books into the TV show- where they changed a few things!
So it really behooves a Douglas Adams Fan, and a H2G2 fan to own ALL these iterations- because they are all different and equally great!
The movie is kind of amusing- but was really doomed from the start, because it is almost an unadaptable property for a feature film. Just WAY too much story to cover in 2 hours!
But I though Martin Freeman was great casting. I was horrified by the casting of Mos Def as my beloved Ford Prefect. I liked Sam Rockwell as Zaphod. I have no idea why they pictured Marvin as a little droid.
(Even in my mind's eye with the radio show, or the books- I NEVER pictured him as a midget robot).
All in all, I wasn't depressed by the movie (no Marvin pun intended) - I had no high hopes for it from the get go.
So no expectations to dash.
P.S. the radio cast was aces, and just couldn't be beat. Carrying over Simon Jones to the TV show was a good thing. Just a few years ago the BBC had an anniversary concert where all the living H2G2 radio cast got together and redid the whole part 1 & 2 (hitchhiker & restaurant at the end of the universe).
You can buy it. It's great!