Today's Avacyn Restored preview card is a very interesting card for many reasons:
I think we can safely say that there is a cycle of angel-esque "hate bears" in Avacyn Restored. First there was Sigarda, Host of Herons and now this. I think they're all going to be legendary 5/5 gold angels with double white in their casting cost and they're all going to have some type of "hate bear" effect. Interesting.
As for the card itself and while I think it's very good, I'm going to go ahead and say that she's one of the weaker angels in the cycle (!). I say this mostly because of her expensive cost, relative to Sigarda. Many times when 5 card cycles are introduced there is a certain range of converted mana costs that they can be and Sigarda has already set the bar at 5cc. Now that Gisela comes in at 7cc, even with the other three having yet to be revealed, I'm going to say that she's on the high end.
Doubling damage and halving damage is not new to Magic (at least on separate cards). Lucky for her, she's in the right colors that she needs to be in to abuse such abilities. Just stick her in a RW midrange deck that has some cheap burn, efficient 2-for-1's and top off the curve with a single copy of her and you're good to go. You could always throw her into a reanimator deck but that's kind of boring and honestly, there are much better targets out there to abuse.
EDIT: "I believe it has been confirmed there will be no black white legendary angel. There are three flights, and we have GW (herons) and RW (Goldnight). The UW will be alabaster and that is it." I stand corrected on the angel cycle. My theory is shot. And even then, my math was wrong. It would only have been a 4 card cycle if I were right because we're talking about gold cards with double white. #fml