I've been looking at these recently - there are some Polaroid branded ones going cheap on eBay. I've not tried one though and I think I'll just buy an Haida ND3.0-II (1000x) for long exposures.

Putting a pair of polarisers front-to-front is HORRIBLE, even ignoring the mechanical craziness needed with reversing rings and stuff. Once you get past about 6 stops, you get very dense and uneven colour casts with funny patterns in them. I'm not sure, but I suspect the problem is the presence of the quarter-wave plate on the input (what was the back of a circular polariser) side. Their effect, being to introduce a 90-degree phase lag in one axis with respect to the other and thereby convert linear to circular polarisation, is strongly wavelength-dependent. A commercial vari-ND would have to be far, far better than combining polarisers manually, even if that's how they work internally.