I'm looking to get an ND filter for when my Voigtlander 25mm shows up. Anyone have any experience with this one? Any other recommendations?

Light Craft 52mm Fader ND Mark II Filter 415 B&H Photo Video

Check out Genus brand also - they work well.

But why get the 52mm? This will not fit most other lenses then you need 2 or 3 more ND faders.

I use a 77mm ND and stop down rings to fit smaller lens sizes down to 52mm.

That would make sense if I bought more lenses. Right now I have the stock 14-42mm. I don't plan on using ND on it because the lowest aperture setting is f/3.5, so it's not like I can really preserve a shallow depth of field on it. As far as the 25mm, its 35mm equivalent is 50mm. Then if I switch on the ETC mode that effectively gives me 130mm. Two great lenses/focal lengths for the price of one. I'm not sure what other lenses, if any I would want. If I get a wide angle lens I wouldn't be looking for shallow dof.

The only 77-52 step down ring I found on B&H is this: Novoflex 77-52mm Step-Down Ring for Ring REDUZIERRNG 52/77 B&H

So the 52mm Lightcraft (or Genus) is $72, and the 77mm Genus + Step down is $135 + $58 = $193. Doesn't seem to make sense to go that route, unless I'm missing something. Thanks for bearing with me while I work this out.

That is not a step down ring.

You can pick up all sizes of step down and step up adapters from ebay for about $5 each. They are indespensable unless you want to buy a set of filters for each lens - not just ND, but UV, polarizers etc.

By the way ND filters are not used just to shorten the DOF. Most lenses have a sweet spot where they perform best (sharpness, abberations) and that is anywhere from F5.6 to F11.... whatever.