Every control deck would want to have two copies of Voodoo Doll. Of course, a class like Mage will benefit the most from it since it can kill the minion right away using its hero power.
But other classes also have the tools to make it work. For example, Warlock has a combo with Mortal Coil, Druid with Moonshine, Rogue with Backstab, Shaman with Zap!, and so on.
Think of it as a 3-mana Assassinate that requires a combo, and you will have a very good hard removal on your hands.