[petrochemist]

I' used a cheap variable ND (~£20 from 7 day shop).
It does give a slight colour cast, but seems to do the job just fine, and the colour is easily corrected.
This one & its partner in my Gallery where taken using it.

BTW variable ND filters are made using 2 polarisers (a linear then a circular) because of the way they work they can give odd effects with wide angle lenses - especially with the sky which will darken more at 90° to the light source than elsewhere. This applies equally to the very expensive models!