Holy Shock is a unique instant cast spell. It can be used either as a "clutch" heal or a powerful "nuke" spell.
It is slightly less mana efficient than [Holy Light] and so should only be used when an instant cast is essential or you may risk burning your mana too fast. [Flash of Light] is the most mana efficient paladin healing spell.
As a spell from the Holy School, using it offensively with [Righteous Fury] can generate needed aggro when off-tanking. A quick jolt of holy energy with at least 160% aggro generation will grab attention.
This ability was significantly buffed in the 3.0 patch so that it can now be a reliable spell in any healing rotation.
Patch 5.4.0 (10-Sep-2013) Holy Shock's mana cost has been reduced by 50%.
Patch 5.0.5 (11 September 2012): Holy Shock’s base damage has been reduced, and its spellpower coefficient has been increased.
Patch 4.2.0 (28-Jun-2011): Holy Shock mana cost has been decreased to 7% of base mana, down from 8%.
Patch 4.0.6 (8-Feb-2011): Crusader Strike, Holy Shock, and Hammer of the Righteous no longer incorrectly grant Holy Power from a missed attack.
/Patch 3.0.2 (14-Oct-2008): Holy Shock (Holy) range changed to 20/40 yards when used on enemies/friends respectively, cooldown reduced to 6 seconds.
Patch 2.4.0 (25-Mar-2008): Casting lower ranks of these spells is now properly penalized like other healing and damage effects.; The healing, damage, and mana cost of this spell have all been increased.
Patch 1.10.2 (02-May-2006): Bug fix: Now guaranteed to crit when used offensively while Divine Favor is active.