I have quite a lot of the film holders and have tried to use them, but for me it is just more hassle than it is worth. Certainly it is interesting to be able to view with the ground glass on the back, especially to see if the taking lens and the focus lens agree on focus or if you want to see what you are really getting with the number 3 Rolleinar, and for me that is enough to justify owning one that I got off ebay for 25 bucks. They are pretty problematic in that you have to be very careful not to fog your film due to the leaky velvet and the soft bendable metal. the film is available by Efke but it is easy to cut down larger sheets. I cut a bunch down from 5x7 with very little waste. One other interesting thing about using sheet film is that the film lies absolutely flat.. so none of that film curl problem that is only mythical anyway.
dennis