those who run them will tell you they are unnoticeable from the driver's seat.
Gotta disagree. I ran supersingles on a Freightliner, and noticed a significant drop in vibration at highway speed.
Consider that when you have two tires bolted together (duals), the slightest difference in diameter, tread pattern (or wear), pressure, etc will result in the two tires fighting each other every inch of the trip.
With a supersingle, you have ONE tire to balance and inflate, and ONE tread pattern.
If I were driving a big truck again, I would run supersingles. I don't expect to run supers on my coach, because I want the option of swapping tires in case of problems.