I have used one for a long time, probably 25+ years now. I don't really like the #90 or the zone vi. The one I use is a darker amber made by Peak of Japan. I don't think it is imported any longer. Tiffen makes one that is good and available. Harrison makes several, different colors: grey, blue, amber. Seems amber is traditional. Grey is for color ( I have a spare one of these. ) Blue is the theoretically correct viewing filter for BW. Since your eye sees best in the greens and films see most in the blues, you need a color that shifts your eye more towards film. Then there is an overall density to allow you eyes to see into the highlights (like sunglasses) and to see what is going to go dark This helps so much for contrast evaluation.
On the one I use, I mask off the filter to match my format size and evaluate the scene with the filter, both for contrast and cropping, and also depending on how far the filter is from my eye, I can guess the proper lens.
A great tool.