Never used one but have talked to others who have and they like em. I'm just old fashioned and like 5 or 7 leaves under me, but there is nothing wrong with them at all. My Son Dan may be replacing the front spring under his RPU with a monoleaf because if offers some height adjustability by stacking the spaces either on top or under the leaf.
There have been some reports of them breaking, but those seem to be isolated and questionable as to what caused the damage.
Don