I've wondered about these, too, and almost pulled the trigger on a set the last time I ordered sights just to see for myself.
I think the Ghost Ring concept sounds good, but have yet to hear of one person that has actually shot *better* with that combo.
I know they are illegal in GSSF competition, which at first led me to believe they were an "unfair advantage", but maybe they were just so bad that spectators were getting wounded???!!!
I would love to hear some reports of those that have tried them. If they like them, why, and if not, what was wrong with them???