Duff has been called a "master of trenchant and incisive English," who "wrote his opinions in a style which bears comparison with Holmes or Birkenhead." A former assistant of Duff, Kenneth Campbell, argued that Duff was "frequently ranked as the equal of Justices Holmes and Brandeis of the United States Supreme Court," and Gerald Le Dain asserted that Duff "is generally considered to have been one of Canada's greatest judges." Other writers have taken a less favourable view, instead arguing that Duff's reputation is largely unearned; his biographer concluded that he was not an original thinker, but essentially a "talent student and exponent of the law rather than a creator of it."