For me, it's all the varieties of Marans. They lay large dark brown eggs, and they are somewhat decent layers. Then again, I suppose many people don't really think of Marans when it comes to FF, as their feet are not heavily feathered, so maybe it would be something like Brahma or cochins.
FF = Poopy, dirty feet. Frostbite in winter because of wet foot feathers. Hindrance to chickens being able to scratch for food while foraging. An impediment when running from predators. All Cons.
Honestly? I can't think of a single pro, other than being something for show...which is a pretty...