I use one of those variable filters. They work. A quirk of mine (and some others, I gather, from reading comments) is that when you dial them to the max, you can see a darker X shape in your image. I don't know if all variable NDs do this. I just use mine a bit short of the maximum.
From what I've read, a variable ND filter is actually two polarizing filters stacked together. When the two are in the same orientation, you get the smallest light reduction. As you rotate one filter, the combination blocks more and more light. ND400 is about 8.5 stops, I think.