I finally got round to seeing this a couple of days ago.
It was very watchable as a movie in its own right, certainly no travesty, but I still came away feeling slightly disappointed. I just couldn't believe how many crucial jokes they had left out. It felt very similar to Terry Gilliam's films which are OK but not as funny or clever as The Hitchhiker's Guide.
I think the main problem is that most of the jokes in the books are in the dialogue and narrative. For some reason the filmmakers decided to strip most of these types of jokes out and replace them with mostly lame visual gags. I know that film is predominantly a visual medium but IMHO the BBC series managed to remain much closer to the spirit of the books on a much lower budget.
Having said that, the friend I went with really liked it. But he hasn't read the books or seen the BBC series so he wasn't aware of what he was missing.