Not easy to activate since you have to have a level 6 monster, but special summoning Dark Magician from your deck without cost is useful. You may want to use if your deck is based on Dark Magician/Dark Magician Girl and cards which support those two monsters.
I just got this from Tea for the first time right after I watched the episode where she used it. It almost makes me wonder if Konami has a way of spying on us. XD
10 times man? Poor guy... I spent over 5000-6000 keys and Tea's drop rate are among the worst. I do have 4 Sage Stones but only 2 Dark Magician Girl. Magic Formula, however, I have ELEVEN!