They look cool
I suppose it's just something that sets you aside from the standard oldstyle magician so many people are used to - I use all sorts of cards, but I get most comments aesthetically about my tigers.
If you play about with the darker side of magic they're cool, I did a walkaround gig hallowe'en at an Eerie Pub Co. (Crazy gothic looking bars!) bar called the Pit and Pendulum here in the UK and they really fit well there!
D.
They look cool. But I myself am completely satisfied with regular bikes and tallys. Also they cost twice the amout of money than regular cards, and in the long run it will be very expensive. So black cards are a no for me.
I used to, but when the edges start to whiten they look horrible.