[font color=\'#000000\']I have one. I'm shooting digital RAW, so I'm not so concerned about setting white balance with it. It can be used to do that, in fact I think it's wonderful for that purpose, if you need to. I tried it several times and I liked the results, but I never relied upon that.
What I now use it for is to test and calibrate my light meters. I do the Sunny 16 test with this disc on my lens. I know my in camera meter underexposes by 2/3 of a stop as a result. I can then take this into account when I set my exposure. I can also shoot a gray frame at different exposures that will at least tell me where, for example, zone V or zone III falls in tonality.[/font]