I love cards!! I use them in all forms of my writing! Feature Scripts = Cards, Short Films = Cards, Picture Books = lots of Cards!!!! I buy as a giant stack of them every time I go to Target, because I know I'm going to burn through so many of these on every project I work on.
For me, the card is where the story starts to take shape. I start with an idea in my head, then I write down everything I can think of about that idea on these little 3' by 5' cards. And I mean everything!!! It doesn't really matter if I throw these cards out later, or if 80% of the ideas will never be used. It all has to come out of my brain first and onto these cards that way I can start to see what I have. Then and only then can I start to move some of these ideas down into a story.