Flawless is a partisan-type meleeweapon which can deal high critical damage for infinite periods of time, assuming the player doesn't get hit. As soon as one takes damage, the DPS of this weapon becomes much lower, becoming one of the weaker melee weapons. Therefore this weapon can be considered high risk/high reward, especially against Elites and Bosses.
"Increases your movement speed for 5 seconds after a critical hit."
Synergies
Damage negating items like shields or certain skills can prevent you from taking damage to allow longer uptime on Flawless’ crit condition.
Skills such as Foresight or Ice ArmorRotG can be used to negate damage and keep up the crit condition for Flawless.
CocoonFF and shields can also work for negating damage by parrying, but this has to be done manually.
Trivia
Prior to v1.9, Flawless shared its in-game model with the War Spear, and the Impaler. Of the three weapons, the icon used for Flawless resembled the model the most. This is ironic, considering it is a halberd, and not a spear. The Flawless is even referred to as the Perfect Halberd internally.
However, it actually much more closely resembles a Partisan than a halberd.