Having played with these for a few weeks now, I totally agree with Seige

in that it should only ever be performed to the same set of people once, but that is a general rule I think that applies to most magic. There is very little magic that I would feel confident doing more than once to the same people.
When I do this trick now I also have a couple of svengali/mirage deck in one pocket and the ID in the other, just on the off chance that they might pick the actual
svangali card. If this does happen it's really awesome. I take the svengali pack out instead of the invisible and riffle through it getting them to pick their card - which is of course the same one thay they picked out when the cards were invisible...
Addmitidly this has only happened once so far, but was very impressive. In saying that - people are just as impressed with the ID trick, but it does means that I can do it again with the same people and show two amazing tricks...
just an idea.
