Wow. I saw it tonight with my daughter. She's not read the books and she loved it. I *have* read the books and once you get over the idea that its *not* the book, its great.
Zaphod is a complete PITA, Ford is underacted, and Arthur isnt grumpy enough. That aside, its a testament to HHGTTG, and is hilarious. Really funny.
The cast is a complete itinerary of British Comedy. Bill Nighy as Slartibardfast is very very good. Its so good you have to wonder that Douglas Adams had Bill Nighy in mind.. Bill Bailey as the whale, Stephen Fry as the book, Alan Rickmann as Marvin. All very good.
So see it. Enjoy it. And observe the backgrounds. There's a lightsaber, and there's an "original" Marvin.
And I'm guessing that there'll be a sequel. I hope.