For me personally, the plot doesn't matter all that much anyway. What I love is Douglas Adams' prose - the plot's mostly just a vehicle for that - and I feel that doesn't really translate to film. The perfect example:

The ships hung in the sky in much the same way that bricks don't.

It's funny. It's succinct. It's very descriptive. It doesn't just tell you that the ships were hovering, it draws comparison to bricks which conjures up images of blocky, inelegant ships, and it gives the impression that the way they're just stationary in the sky is somewhat unsettling or surreal. I think it's quite impressive how much such a short sentence manages to convey really!

Translating it to film, and having shot of some blocky, inelegant ships hanging in the sky, doesn't manage to capture the same humour or feeling that that short sentence in the book does, at least for me. And it's the same throughout the whole series, but that line is probably the easiest example to bring up. Some books translate really well to film and the imagery in the film ends up being far better than what I could imagine myself on the fly, but that's not the case with Hitchhiker's Guide at all.

The Hitchhiker's Guide radio series has a fair amount of narration so the prose still shines through in that.

I had similar issues with the various Dirk Gently adaptations, too. And I find I have the same issue with screen adaptations of Terry Pratchett's work for similar reasons. Without Adams' or Pratchett's wonderful prose, it often tends to feel very B-movie-esque to me.