Very pleased with the combination of the Vari-N-Duo with my 5DMkII when I had a chance to try out the combo recently. It's not cheap to buy in the UK - it cost me £270 (about 430$) and unfortunately I got clobbered for another £49 import duty a couple of weeks later. Very quick and easy to use though, just twist the ND filter to the desired effect between Min and Max (roughly 2 and 2/3 to 8 stops) then turn the whole front to change the CPL effect. It's larger than I thought and on my 24-105mm it vignettes to about 40mm - it's better on the 24-70mm though. I can also screw my Cokin Z Pro filter system to the front and add ND grads. It's definitely a keeper for me. See my test shots:
http://www.pbase.com/making_trax/image/118552473/large.jpg
http://www.pbase.com/making_trax/image/118553576/large.jpg
http://www.pbase.com/making_trax/image/118552897/large.jpg

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Steve
http://www.shrewsburyinsoftlight.com
http://www.stevemorrisphotography.com