So, let's skip the preamble and get to the most apparent issue: colour cast.
Above: Fader-ND removed (T2i, Sigma 50mm f1.4, handheld, liveview histogram to set exposure)
Above: Fader-ND attached, set to "Min" (T2i, Sigma 50mm f1.4, handheld, liveview histogram to set exposure)
Above: Fader-ND attached, set to "Max" (T2i, Sigma 50mm f1.4, handheld, liveview histogram to set exposure)
Above: Side-by-side comparison makes the colour shift apparent.
Process: All the above photos are pulled from a .JPG captured at "Fine" resolution, dropped onto my drive, opened in Photoshop CS4 (PsCS4), resized to 10% of original, and saved to web as 100% quality JPG. A lot of variables and points of failure, but since all images went through the exact same process any differences should be the result of either my shooting or the filter.
Right away we can see the green cast to it. Others have pointed it out, so it's not a flaw in my particular fader. Other reviews have mentioned the same effect on the Singh Ray vari-filter, so I'm willing to bet it's a problem with these types of filters.
Is the green cast a deal breaker for me? No.
When the choice is this for $125 US vs. $400 US (at least ) for a matte box with ND filters, I'll take this and deal with the extra steps in post until I can call myself enough of a professional to make that leap to the proper filtering system.
For still shots I can figure out the amount of deflection required in the green curve and then just batch process all my shots in PsCS4 to remove the green tint. As a neophyte to HDSLR video shooting, I don't yet know how to colour (YES, colour with a "U" – ya' got a problem with that <g>) correct video footage, but I sure Premier Pro / After Effects CS4 has that ability. One way or another, I can compensate after the shoot to correct the colour issue.
Some of the other product/purchase details: it seems well constructed – metal and glass, tight tolerances, smooth rotating action. The purchase was pain-free and the shipping from Hong Kong was quicker than I expected.
One of these days (i.e. if someone actually asks me to) I'll get around to planting the camera on my tripod and doing some resolution chart shots to see if the Fader-ND impacts sharpness. Until then, I'll be happy with what I have and dream of bigger and better gear as my skill level increases.
If you want more info/shots, throw down a comment below and I'll see what I can do.