Anything 3+ is good for cost looking at history. But the question is for anything bigger, his mage getting enough big spells to support the archetype? What would you run in addition to compliment the maybe 15 potentially viable bigger spells?
Remember when one of Warlock's unique trait was that it has "Deal damage to ALL minions removal"? Yeah, that's gone now. Priest has a better Twisting Nether and Mage and Warrior have better Hellfire / Felfire Potion...
Btw, this is already good if it hits a 3 mana spell. That means apart from Frostbolt or Glyph you'll always go even.
This seems really unfair, you've no need to run things like Volcanic Potion with a card like this. If you just stack your deck with big spells then it becomes more powerful than aFlamestrike on 5 mana and you only need to run single target removal as this will act as your board clear.
Good board clear vs aggro because only need 3 dmg to wipe them out. But you cant clear the board against big minion unless you are lucky with 7+ spell. Mage need samethibg like brawl or twisting neither. So it is a good card but cant really improve mage weaker matchup much.
In a big spell mage this is an amazing card and time will tell if that deck sees play. But as long as Primordial Glyph is an auto include there will always be a risk of this being a more expensive Volcanic Potion. But even pulling Fireball makes it a discounted Flamestrike, and the possible high roll is super worth it... for me, its just this side of playable- almost very good.