I have this lovely Filofax Ochre Malden iPad Cover which is made of the loveliest leather. It has such a nice texture and colour and has been pretty hardy. I have had it for about 7 months. I use a Logitech keyboard as a cover for my iPad. This means I can use my iPad to type long documents, and leave my Macbook at home.
So I use this cover to hold my documents. It keeps them together, sharp and pointy objects in my bag don’t get caught up in the pages of the books, and I can slip other bits of paper in there as well.
It could, as a pinch, be used as a Filofax sleeve. I don’t put my Filofaxes in any protective covering in my bag so I have not tested it.