Not bad! It did what movifications always do: cut out a lot of important stuff, and provide a vision that doesn't necessarily match what fans might have had in their own heads. But Mos Def definitely worked, I thought, as Ford Prefect, and Arthur Dent was suitably clueless, and the Infinite Improbability Drive was, well, Infinitely Improbable. I remembered a number of those Guide entries, and the style in which they presented them was quite elegant. Marvin was depressed. The Vogons were too bloated, I think, and Zaphod Beeblebrox was a little overdone.
Yes, there was a romance, and the romance was campy. But tHhGttG is campy. So I think it worked out too, but perhaps not well enough to fit in with sequels. It wasn't the best movie I've ever seen, but I don't regret spending the money: I had a sufficient chuckles to make it worth it.