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Will a variable neutral density filter produce similar results to a fixed neutral density filter?

I have just spotted an "adjustable" ND filter on eBay. The way it works is basically two polarised disks which rotate to allow more or less light through.

Although this technique seems solid enough at first glance, the fact that I had not heard of this before makes me wonder if its really just a nasty hack with unforeseen consequences. Are these filters comparable in quality (of the result) to a regular ND filter?

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No they are not comparable. They cause weird cross-polarizing effects and color-shifts while a good fixed neutral density filter has a very uniform effect across the image.

Although a briefly considered one for its flexibility, I wisely decided against it and now use a 16X and 400X ND filter for my needs. I still have a 8X one which causes too much color-shift but at least it is global, making it easily correctable in software, unlike variable filters whose effect differs depending along with angle.

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There is a huge difference in optical characteristics depending on how much you pay.

http://strobist.blogspot.com/2010/06/using-nd-filters-to-kill-depth-of-field.html

Quoting: "If you want optical quality, durability and continuously variable densities, there is one option. And it is expensive.

The Singh-Ray Vari-ND is the ne plus ultra of ND filters. It gives you a "dial-in" setting of anywhere from two to eight stops of neutral density -- that is actually neutral. And it is sharp, too."

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