Great card and a must-have for Justice with a spy character (Mockingbird, Black Widow, Spider Woman, etc). The best part about this card is canceling high priority encounter cards like "Shadows of the Past". What's nice is that it triggers in both modes, so you have a security blanket in place whether you're hero'ed up or cooling down as your alter-ego. It is balanced out by having to draw another card to replace the one you just canceled, but that is why you save it for that really bad encounter card. Once you know "Shadows of the Past" is out, you can risk using it on other potentially bad cards. Remember also that it cancel's "ANY" encounter card including side schemes and attachments unlike "Grappling Hook" which can only works against "Treachery" cards while in hero mode as well. Plus, you can put multiple copies down since there is no limit for the amount in play other than the maximum of 3 in your deck.
As always, it synergizes better with Black Widow, since it has the "Preparation" keyword.