I don't know how long they've been up, but in case anyone's not aware yet (I sure wasn't), here's the rundown: They suck. They're nothing compared to the Soul Dew, and quite possibly Life Orb and Choiced variants as well.
Mega Latias: +20 attack, +30 defense, +30 special attack, +20 special defense
Mega Latios: +40 attack, +20 defense, +30 special attack, +10 special defense
And they both keep Levitate too. Talk about underwhelming.