Much of Koestler's work was completely out of step with mainstream views. He did not merely arrive at different answers to common questions. Anyone can do that. Instead, Koestler answered, or tried to answer, important questions that others were not even asking. This is often a sign of true creative genius.
The result of this originality is a uneven set of ideas and conclusions. Some of his ideas can be appreciated as brilliant, as his work on creativity. Some ideas challenge us to readjust our thinking in order to grasp their importance. And other ideas are just nonsense. But taken as a whole, they are well worth serious consideration.