He loved Empire. Was on the edge of his seat most of the film, was freaked out by the Degobah cave scene, loved the Hoth battle, and couldn't watch when Han was being turned to carbonite. He was floored by the suggestion that Darth Vader was Luke's father. Mouth agape.
That said, I think I know where Lucas was coming from with the prequels. First thing he asked afterwards: "How did Darth Vader become a bad guy?"

