I suspect it is a bit of both. I read the books, but a long time ago, and I didn't love them; and it worked fine for me. I gather there were people upset at jokes that didn't make it in, at changes, at things being cut out due to time, etc. And I'm not sure I'd've enjoyed it as much without a frame of reference. People who haven't read the books may've been expecting it to make sense, which is not really a HHGttG sort of thing to do, I fear.

I'm not sure there's much reason to see it in theater, though. It doesn't do the sort of special effect or presentation that's really best on the big screen or anything.