I just think that the card's value is greatly diminished in decks that end up usually only playing one card per turn like Control Warrior or Ramp Druid, and gets exponentially better in decks that have tools to both quickly draw and spend cards like miracle, combo druid, handlock, zoo, or midrange hunter. Basically getting stuff one turn earlier isn't fantastic, especially if you're already behind. He's still very good, better than some general-use legendaries like Loatheb, but I don't think he has as much universal utility as Boom. He doesn't outclass the other 5-6 mana cards for me as much as Boom outclasses the other 7+ mana cards, but I need more time to see what he can do in actual play.
We'll see, though. I'm sure he has some wild applications that none of us are savvy enough to think of, and I'll definitely focus on getting him to work since he's the most interesting card in the set.