I think the reason why people don't like the Dirk Gently books is tht they expected them to be of the same kind of silly, pythonesque (if you will forgive me for creating that word) humour, which they weren't. The humour is totally different, more mature, but it is still there.
And I definitely recommend to read "Last Chance To See", Adam's book about nearly extinct animals. It is not comical at all, or very rarely at best, but absolutely worth reading.