A brand new planeswalker for your viewing pleasure, courtesy of Avacyn Restored:
My very first impression was: "Wtf? when did the moonfolk get to Innistrad?"
My second first impression was: Hm... I think she can be very good. She can freeze down permanents and/or potentially gain card advantage if cast under the right circumstances. Not only can she protect herself, but she can also replace herself. Perfect.
My third first impression was: God, her ultimate ability is funny. Not that is says "a card" from anywhere. This includes instants or sorceries you cast from your hand. Once you go ultimate with her, all of your spells basically have a buyback cost of zero and the game is over.
She definitely wants to be in a control deck and I'm going to go ahead and say she would love to be in a deck where she can play together with some planeswalker buddies, specifically Gideon Jura. Instead of "wasting" one of her turns protecting herself, why not use Gideon to protect her? Drop a Gideon, force the attack, drop a Tamiyo, and reap the rewards for your troubles. Perfect.