Brain Freeze

Brain Freeze is a Mage talent in the Frost tree. It makes the Frost Mage's spells which cause chill effects (such as Cone of Cold and Frostbolt) have a chance to grant them an instant-cast Fireball or Frostfire Bolt at zero mana cost.

PvE
This talent is often part of deep Frost Mage PvE builds. Despite that Fireball being untalented in an average Frost build, its instant cast time puts it on par with Frostbolt (a Frost mage's main nuke spell) in terms of raid DPS, actually increasing it very slightly. Where this talent shines, however, is in mana-efficiency, allowing Frostbolt-level DPS to be dealt at no mana cost.

The Frost tree, however, is already the most mana-efficient stand-alone tree between the three Mage talent trees without this talent. Its small DPS increase is surpassed by that of outside talents in Arcane or Fire (such as Ignite and Focus Magic), so that it's often not part of streamlined single-target damage dedicated Frost builds.

In the end, whether or not taking this talent is up to the player's opinion of whether Brain Freeze's soloing and PvP convenience or an extra few percent of raid DPS are worth more.

PvP
This talent allows for great mobility in PvP, giving the Frost Mage even stronger burst potential. It is recommended that the instant Fireball be saved for a non-Frozen target, as Ice Lance isn't usable to great effect on those. The chills on frost damage spells such as Frostbolt, Cone of Cold and Blizzard (through Ice Shards) can trigger it.