Fun idea. It lets players feel as though they have more control over the narrative. To be used with caution, however. Not every card will work with every session or game, and the GM should only use them if they are confident in their ability to think on their feet. Also, they have the potential to change the course of planned events, to plan accordingly. If used properly, it can definitely make the game more enjoyable for everyone involved.
[4 of 5 Stars!]