Vari ND's suck, as mentioned/implied, and i myself have tried them. Although the expensive ones are multicoated, they also give you a cross hatch, that you can't get to because they have a physical stop. The cheap ones, just let you keep twisting and twisting.
If you want a big stop, there is a nd 32 on ebay, that i bought that does work, although it gives a strong blue cast, while not a big problem in b/w, it wouldn't work with color film.
the expensive big stopper (LEE's big stopper), also gives a strong color cast, but they send you 'settings' for white balance/color temp if you use Dig*tal. One of the reasons you often see these photos with 10 stops in black and white, is the strong color cast these nd filters give... i think i read somewhere that it's because they aren't true nd filters (i don't remember the rest).
anyway,
don't get vari-nd filters
do get nd32 equivalent to 10 stops, by my tests (don't forget reciprocity)