Garrote is used more often in raids because of the fact that most mobs in raids are immune to [Cheap Shot].
For targets not immune to stuns it's best used after opening with Cheap Shot (unless at full CP, at which point it's best to use [Rupture] and then Garrote).
Garrote's silence effect makes it a highly viable opener versus casters who can shake off a stun or immediately react to an ambush with a detrimental effect that can turn the fight around, such as [Blink]. See silence effect for general tips on usage.
Patch 7.0.3 (2016-07-19): Now only available to Assassination rogues. Can now be used outside Stealth, but only silenced if used from Stealth. Damage increased by 1400%.