Nothing incites the killing urge like the taste of one's own blood.[1]
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.
Patch 7.0.3 (2016-07-19): No longer triggers [Enrage]. Cooldown increased to 1 minute (up from 30 seconds), duration reduced to 6 seconds (down from 10 seconds).
Patch 5.0.4 (2012-08-28): Instead of causing extra Rage generation when taking damage, now generates 10 Rage and increases physical damage done by 10% for 6 sec.
Patch 3.2.0 (2009-08-04): Tooltip clarified to indicate this ability breaks existing Fear, Sap, and Incapacitate effects.
Patch 3.0.2 (2008-10-14): Berserker Rage is now usable in all stances.
[Outburst], 
[Warlord's Challenge]
[Sap] and Incapacitate effects and removing any such effects currently afflicting them. It lasts 6 seconds, and has a 1 minute cooldown.
[Repentance].
