is anybody out there using an expodisc to white balance?

i'm considering getting one to help me WB on the fly while shooting documentary footage. (i'm trying to get away from using a card.)

i'm curious whether the expodisc will perform in mixed, available lighting situations, and whether one can use it in a quick-and-dirty fashion--by simply pointing it at the subject and setting the WB.

here's the link to the product, for anyone who's interested:

http://www.expodisc.com/

thanks,
phb

I think you have to point the Expodisc at the light source, not the subject.