Skill is primarily used at the beginning of duels or when initiating mobbing when a player/mob is out of immediate range of regular attacks with daggers.
Fast Attack is also one of the few skills which attacks instantaneously (damage is made almost straight away, rather than after a long graphical animation delay) and is also can cover a distance in the same way too. It is also noted that there are cases in which you can use this skill to evade a players attack when dueling as there is a brief moment of 'teleportation'.
However if an enemy is already on the floor (or falling) or in the process of moving the skill has a high chance to miss, so it is advised that if possible wait until the enemy is in a standing still position before initiating this skill.
Note: there has been discusion in regards to Fast Attack having stealth properties as the skill description suggestion in 'stealth' suggests it adds a bonus when being used under stealth. Generally this is revoked as not being the case and thus being miss leading.