I'll second all other comment-writers who point out the witty humor and Errol Flynn's still nimble swashbuckling at 38 in this very entertaining film, but I'd also like to point out another feature of Errol Flynn's performance (also applicable to many other of his films):
He is also good at delivering more serious lines in exchanges with the villains (in a very believable, underplayed manner) and, for that part, more sincere dialogue with the beautiful ladies.
I think this is an often overlooked part of Errol Flynn's charm and why many of his films, although light in subject, make for truly timeless pieces, just like The Adventures of Don Juan.