Upload & Sell: Off
I tried the GNDs and got decent results only, as mentioned above, on scenes with a clean horizon. Even then, I got the best results when I hand held the filters and moved them during exposure to reduce the obvious effects.
Still, I never got the satisfying results that manual blending gives, so, after realizing I'd never use them again, I sold the GNDs.
As others have stated, manual blending can give much more natural and pleasing images, especially when done carefully. Yes it may take a little time, but with practice it's not cumbersome at all.
(The only filters I now own are a CPL and a Vari-ND, to get longer exposures, especially for scenes with water.)