I never owned one, but my wife did before we were married (called it "Betsy") and many friends did so I got to drive quite a few miles in them and ride more. Advantages were low initial cost (brand new for $1500 on the Portland docks at one time), great for the time fuel economy, utter simplicity making them very easy to repair and work on, and like the other folks say surprising ability to go in many unexpected places (they would also float).
We had them on many logging and range roads out west and they were able to go places only 4WD went. Plus if you put them in the ditch they were light enough to make them easy to get back on the road.
Biggest problem (among the several already mentioned) was the defroster. If you were the passenger that was you. If you were alone you got good at wiping the windshield and driving at the same time.
And with minimal repairs and a little babying they would go and go and go. As the passenger I was also often the starter; a quick push and popping the clutch usually got you going.
One of my dad's friends reviled them, calling them "pregnant roller skates" until someone offered him a deal too good to pass up. Pretty soon the family had a whole bunch in various states of repair. They were into hunting and mining and so introduced a lot of remote Oregon to them. I can see how they would be a great vehicle to chase trains; lots more money left over for film and camera gear.....
Timothy