I've used the Singh-Ray Vari-ND for many years -- it's an extremely high-quality piece of equipment, and I wouldn't part with it for love nor money.
In comparison with conventional ND filters - frankly - there is no comparison; image quality is identical but the variable attenuation simply blasts conventional ND filters clean out of the water (it isn't even close). With a Vari-ND you just dial in the shutterspeed you need for motion effect, dial in the aperture you need for desired DoF, and then just adjust the Vari-ND to balance the exposure.
Keep in mind also that quality landscape is invariably shot in the golden hours - and in the golden hours the light levels change constantly. Tweaking the attenuation to accommodate that is easy with a Vari-ND, but MUCH more work with separate filters if you're still trying to control both SS & Aperture.
A work of warning though, in as much as the Singh-Ray Vari-ND is superb, there are others that aren't as good (although they are much cheaper).
I've written a few articles for the Singh-Ray blog that may give you a better idea as to how I use them ...
http://singhray.blogspot.co.nz/searc...lin%20Southern