Vortex Media "Warm Cards" for custom WB

[Omaroo]

Hi all - long time, no type

It's been a while since I've been here, and have bought a 40D in the interim. I'm going through the features of the camera and have really become interested in custom while balance.

Vortex Media in the States has a set of white balance reference cards called "Warm Cards".

http://www.warmcards.com/WC_PHOTO.html

Has anyone used them to good effect? I tried to do a search first, so that I wouldn't be asking a same question that's already been asked, but it seems to be policy for members who haven't been on here for a while not to be able to search.

TIA