A 19th hole is a neat idea if there's a natural hole present, and a reason to incorporate such a hole.
I recall Tom Doak mentioning Stone Eagle's 19th, for example, was "there". In other words, it was a somewhat natural hole worth incorporating into the design of the course that also helped break-up a relatively long trek from the home green to the clubhouse.
Knowing the developers wanted a unique golf experience, I looked at the possibility of a 19th hole at Sagebrush, in British Columbia, over the past year. There were a couple decent possibilities, before the 18th green was relocated, particularly in light of the trek, up a hill, to the lodge. I thought the 19th hole would provide a "reason" to play up the hill... to get to the lodge.
Anyway...
Scarboro Golf and Country Club has an interesting, and attractive 19th hole. One of Canada's most impressive clubhouses, and a wonderful specimen tree provide an unforgettable backdrop. More interesting, the green is a leftover from the original George Cumming layout at Scarboro. Course re-designer, A.W. Tillinghast (c. mid-1920s), used it for his bye hole; which bridges a gap between the 18th green there, and the clubhouse.
Scarboro's is a tough little 19th hole. It's only about 115 yards or so. But the green is very small, and severely tilted. The green's attached to the practice putting green, too!