Meh. 10 cost cards need to be game winning to see play. I just don't see it.
Not a terrible card. But doubt it will cut the mustard in a competitive deck.
Note: I say this when only 8 cards are revealed so I might be wrong if a strong spell to combo comes out.
Here is the thing--it isn't actually a 10-cost card in a lot of decks. In a lot of mage decks you end up playing Flamestrike after reaching 10 mana and then pinging face or playing a small minion because you don't have much else to do. Think of this as a 4/12 body with a soft taunt on a Flamestrike or Blizzard (remember, it replaces a card in your hand when you play it, so it only expends one card from your hand despite letting you play a free spell), which is SO much more value than what you otherwise would be playing for that leftover mana.