Hi ISO,

I do not have direct experience with the type of variable filters you are talking about. I've been using B+W screw-in filters with step up rings. As I've become frustrated with the filter jamming in the step up ring and with vignetting, I am trying out the Cokin Z-Pro system (haven't shot much lately, but I like it so far). One thought, and potential benefit of rectangular filters, is that you can stack them which allows you to "vary" the ND of the overall set up. The Z-Pro holder allows you to stack up to three filters so you could stack a 1 ND, 2 ND and a 3 ND to get 6 ND...etc.

You may also want to do a few searches as there has been several threads discussing ND filters. Here are a few I found that may be helpful:
http://community.the-digital-picture...ead.php?t=4870
http://community.the-digital-picture...hp/t-3824.html
http://community.the-digital-picture...hp/t-4102.html

Hope that helps,
Brant