The filter you linked to is a 52mm filter so having one would depend on the lens. Personally, although I have a couple of ND filters they fit four of my Hasselblad lenses, I don't use them very often. B+W has seven ND filters with densities from 0,3 to 6,0 with a range of 1 to 20 stops. Stacking polarizers as mentioned might cause vignetting as well as the other issues mentioned.
I'm not looking to stack polarizers. I was just inquiring about these specific types of variable neutral-density filters. Sounds like they have issues at any price point I'd be willing to spend. I was just looking to see if there was a way around having to carry multiple filters, and to avoid using my Lee gelatin filters with my Rolleiflex.
That's good, i didn't want to steer anyone away from experimenting, but stacking polarizers was a def no go.. and from our experiences, neither are vari-nd filters... and sadly that leaves one with single stop nd filters. I recommend you get a 2 stop. which is good and another stronger nd filter (not recommended for color, even lee big stopper has a disclaimer on it's website) for several minute long exposures which are fun. just my 2 cents.
I have one. It's a Hama 2-400 vario ND filter that I have. It allowsw one to vary the amount of optical density added. I provides from 1 to 8 (or 10?) stops of density.
I haven't used it, nor have I had the need! I'm still planning to, so who knows.