Jeff, it is easy to do...but one has to maintain some level of mental sharpness while using it. My last time I used it was coming from Yosemite -- stopped by a river and photographed sunlight on bare alder branches...would have been a great image. But I composed the shot to use the bottom half of the GG...but slipped in the modified dark slide to expose the top half. Did it again when I turned the film back 180 degrees. So the neg had two images of the wrong part of the scene and was not well focused. First time I made that mistake! I was pretty burnt from giving a 5 day workshop in Yosemite and just was not thinking as clearly as I should have!
That is true - it does take a certain amount of focus or mistakes can happen. I use a piece of tape on each of the 4 sides and remove them when it is exposed - that helps a little for me.
Using a roll back is the easiest though. I've moved to the DaYi back, which I got on flea-bay for $200 or so.