If I shot primarily landscapes then a Vari-ND would not be my first choice for a filter. I do shoot portraits however and a Vari-ND fits my needs better than a regular ND filter. My biggest concern in whichever Vari-ND I decided on was which would give me the least amount of color shift at the greater densities and could the image sharpness not be degraded to an unacceptable level? My selections came down to the Singh-Ray, Heliopan and the Schneider versions.
Why did I finally go with the Schneider? Not only are the filters top notch but I also have access to many other options with the Lee kit. It may be a little more cumbersome having the Lee filter kit at the end of the lens but I don't see any image degradation or sharpness loss in my images at around the 5-7 stop range I find myself shooting at when I'm outdoors. Pricey, yes of course but I see it as a necessary tool in my bag that's just as important as any of my lens.