I saw them live once, and thought the show rather contrived, too pat. But I've got several of their albums (even bought the 'Live' series on CD recently), and they did great versions of many songs, trad or in the trad vein. What I didn't care for was their idea of 'humour', whether in patter or songs. But then, I've never been a fan of Billy C. either ...
There is no doubt that they did a lot for folk music by introducing people to it. They were for Scotland what the Clancys were for the States, the Dubliners for Ireland and the Spinners for England.