I played 2 Gideon and 1 Tamiyo (all I had) in UW control at FNM, and it rocked the house every time it hit the table. Using Tamiyo to tap the biggest threat and having Gideon absorb the rest puts aggro in quicksand.
She was played against me last night at FNM. She's the real deal. Between using her +1 to tap down my creatures and -2 to draw cards, my opponent was drawing up to five cards every few turns. It was crazy.
I've been playing a Bant Superfriends with her and loving it. Gideon + Tamiyo has just insane synergy. Use her plus and his -2 and you can destroy any creature. Use his +2 and her -2 and you can draw for each creature they have while letting you get free damage in. (And with my deck generally draw into a DoJ/Terminus from the extra cards.)
She is the real deal and I'm happy to have picked up my four, though I'm not sure I'd run four of her in any of the builds I'm looking at current. I'm considering a U/R or U/R/W build as well featuring more board wipes from Whipflare/Bonfire.
What's great about Tamiyo, and this has been said before but I can't stress it enough, is that she answers whatever you need her to. And if she can't answer it, like say Geist, she can probably draw you into an answer. I love her versatility.