Nothing incites the killing urge like the taste of one's own blood.
Berserker Rage is a level 40 warrior ability that Enrages the warrior, rendering them immune to Fear, [Sap] and Incapacitate effects and removing any such effects currently afflicting them. It lasts 6 seconds, and has a 1 minute cooldown.
Berserker Rage is usable in all stances, and should thus be used as needed: to break fear, sap or incapacitate effects, or to generate extra rage from damage taken when under attack (tanking, PvP, solo).
The immunities can help immensely in PvP combat, where any class who can uses fear effects often. Carefully timed fear immunity can really ruin a Warlock's day, and the other immunities help defend against several Rogue abilities as well as [Repentance].
While it may be preferable to save Berserker Rage for when it's needed most, with a 6-second duration and a 1-minute cooldown, it is possible to have Berserker Rage up 1/5 of the time. In situations where its immunities are of little use, this makes the most of its rage generation effect.