I don't play with this card much any more, but when The Black Riders first came out I used it frequently. It can be a game changer if circumstances are perfect, but it requires some careful planning and a little bit of luck. Imagine dropping an ally Beorn, Glorfindel, or Gildor for free!

I mostly play multiplayer now so I don't play this very much because it becomes difficult to make sure the Elf-Stone triggers during the correct players turn and that the player still has a high cost ally in his hand.