Spell Hunter and Deathrattle Hunter archetypes will benefit greatly from Nine Lives. Since there are already lots of powerful Deathrattle cards in the format, this spell's value will only grow with time.
Even if you take something as insignificant as Shimmerfly and discover it using Nine Lives, you still get an additional minion and a spell for only three mana. That is a fantastic deal.
Similarly, this card will be incredible in Arena, although the epic rarity makes it a rare find.