I was almost ready to order the Fold-A-Cover, but turns out the standard model everyone sells does not work with a dually. I called the company to ask about this, and they told me that their brackets don't work with the Chevy dually bed rails (no idea about Ford or Dodge), and that they had just come up with compatible brackets for a higher price (an extra $30). My concern is that these new brackets were so new, I quite frankly didn't trust them, and I don't like paying huge money (for the cover and the new brackets) to be a beta tester.
I ended up purchasing the Extang Solid Fold. So far, am very happy with it. I haven't towed with it yet, but will tomorrow, so hopefully that goes as smoothly as can be.