well, for the effect you will have to shake the card or else, if held in place, people might see what's going on.
Also lighting may be an issue sometimes, but u can get around that easily by creating shade or moving the gimmick more. But the gimmick is very invisible to the naked eye IF you wave the gimmick around enough.
When I first performed it, I did it for a group of people in a classroom so i was scared as hell that the gimmick would flash, but, I figured, as long as i move it enough, no one will see it, and no one did.
I don't know how close up your gonna perform this, but I think you're pretty much golden if you got the lighting and the movement down.
I think as long as you wave the gimmick enough, no one will catch you.
Also, the gimmick itself is gnacky, you'll have to get used to it; and if not used properly, it will wear out (but i have a solution that can make it last a bit longer).
Overall, it is a very powerful, visual effect which WILL FREAK PEOPLE OUT, or just make them really confused. It gets great reactions.
P.S. if you DO buy it, watch the DVD before you play around with the gimmick b/c wayne goes over how to treat the gimmick, trust me, I learned what happens the hard way. And I can suggest a way to make the gimmick last a bit longer.