Despite a lot of sniping in the media, I believe Juan Carlos will be remembered fondly by history. He could have allowed military dictatorship to continue in Spain but stuck to his line and ushered the country into the modern era.
Handing over now, however, is something of a hospital pass. Felipe must deal with calls for a Republic and with Catalan (and other) attempts at secession, at a time the people are still suffering from austerity.
Good luck to him.