Zone VI viewing filter: useful?

I have 2 Zone VI viewing filters, both 35mm and 6x6. I noticed they can fetch over $100. I'm wondering if people feel they're really useful.

I don't need to visualize composition or framing, but I'm guessing that they might be useful for checking to see if filtration is needed. for

example, to check whether the tones of a subject and the sky behind are merging or have enough contrast.

 

Any opinions?

I also have two -- 4x5 and 6x6. I've never found them very useful. If they are really selling for $100 or so perhaps I'll be putting mine on the market soon!

I find my Zone VI viewing filter very useful for visualising how scenes appear in black and white, and whether filtration is needed.

I bought one many years ago, and still have it. I never found it particularly useful -- in fact, it is the kind of thing that seems to make things easy, but actually prevents you from learning how to do it yourself. After about a month, I stopped using it, and used my eyes and brain -- a much more effective combination. One of the most difficult things you have to do in B&W photography is to learn how to see a setting in monochrome -- learn how to do it yourself, with no aids, and you'll have made a major step.