Irreducible complexity and the cilium

Has anyone read Darwin’s Black Box by Michael J. Behe? If so, what do you think of it? It helped to launch the intelligent design movement in 1996. He purports that the theory of evolution does not account for the molecular structure of life. Darwin himself knew that his theory of gradual evolution by natural selection carried a heavy burden:
“If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.”

The fact that the cilium ( some cells swim using it) is irreducibly complex should surprise no one. The question is, how did the cilium arise?