I think the message is more important but some messengers just have particularly good messages (if that makes sense).
For example I love Terry Pratchett and every book I've read by him (and I've read most of them) I've loved, so when I see one I haven't got yet, I buy it, knowing that I'll enjoy it.
But, at the same time, if I pick up a random book in a shop, by an author I'd never heard of and the blurb sounded really interesting, I'd still be just as likely to buy it.