Very. Barkskin is our primary source of damage reduction in front of high spikes / high foreseen incoming damage.Originally Posted by Puyobwoc
If you are in a fight with predictable extra damage, you should pop Barkskin right before it, so that it mitigates the incoming burst, and most importantly it's less of a sudden jump for your healers.
Quick examples: Maexxna when Enraged, right before the stun; Anub'rekan, right before the Swarm; Malygos when sparked; Sartharion + drakes in the most critical moment; Emalon, right before the Nova; Steelbreaker when/if he gets the rune; Thorim when you get Unbalancing Strike, and so on.
Another situation is where the high damage is constant (Patchwerk OT, for example). In that case, you will want to chain your tank cooldowns so that you always have something up. In this, Narkskin is a great help due to its low CD.
Get confident in how you use it, and remember that it's a very precious cooldown, worth employing in every fight.