So, clue us in on what cameras you are testing.
I needed the dummy rolls primarily for an Ikoflex Ia with 75mm/3.5 Tessar. I picked it up for $25 in good user condition (excellent plus optics), and want to check it out thoroughly before running good film through it.
With all the interlocks, etc on this camera, you basically can't check much without film in it. And it didn't come with a take-up spool. Reading about it on the net, it looks like many people think it's overly-complicated to use, so I want to check that out also.
Also want to practice loading and shooting my Ansco Super Speedex--it was CLA'd by Ken Ruth a few months ago and I want to take it to Europe with me this fall. In the meantime, I need to get very familiar with it and get used to 6X6 composition. So, I'm going to carry it around with the dummy roll and do a lot of phantom shooting. Of course, I'll run some real film through it as well.
As you can tell, I'm new to medium format, though not to film--I've been shooting 35mm since the early 1970s.