Zilliax wasn't an OP card either. You never felt that bad when he was played against you.When it's an auto include in the majority of decks though I don't think it's a very healthy card for the game.
I feel fairly bad seeing this card played in Warlock. It's just the near certainly that you're going to have to deal with 15+ mana being cheated out on turn 10 through Y'Shaarj... I'm not sure I like decks having the ability to tutor high impact legendaries. Paladin has a similar deal going on with Liadrin. It's definitely a card that disproportionately favors archetypes that already have very good cards.