Yep, John Jacob had a unique approach, but I have to say he has a beautiful voice. Go to the site Don lists above and see what you think. I think his singing style has more in common with the high tenor singing associated with medieval sacred song than anything I've heard elsewhere in folk.
Let's put it this way...if it was between Niles and Glen Yarborough's nauseating tremolo voice, I'd take Niles every time.