The mana Innervate saves the target is significant, especially since the spell's cooldown timer is only 3 minutes. It is beneficial to any mana user. The short cooldown means that if you do not need it, you can cast it on another mana user in your group and have it ready again in a short amount of time. Saving Innervate for more than three minutes means you wasted a very large amount of potential mana.
It is a great solo tool - it lowers downtime by replacing a drink, or it can give extra free spells in a tough situation.
In five-player instances, it can be a good idea to save Innervate exclusively for the healer when they are low on mana. When playing as the healer, do not hesitate to Innervate early and often. Just make sure to save it when nearing a boss fight where you think you might need it.
Patch 5.4.0(2013-09-10): Now restores mana based on the Druid's Spirit.
Patch 5.0.4(2012-08-28): Now causes the target to regenerate 10% of the caster's mana instead of 5% of the target's mana. Additional self mana regeneration reduced from 15% to 10%.
Patch 4.2.0(2011-06-28): Now grants an ally target 5% of his or her maximum mana over 10 seconds, but still grants 20% of the druid's maximum mana over 10 seconds when self-cast.
Patch 3.2.0(2009-08-04): Duration reduced to 10 seconds, and cooldown reduced to 3 minutes. This means each use of Innervate will give half as much mana as before, but it will be available twice as often.
Patch 3.1.2(2009-05-19): This ability has been redesigned to grant 450% of the casting Druid's base mana pool to the target over 20 seconds.
Previously based on spirit.
Patch 3.1.0(2009-04-14): This spell no longer costs mana.
Patch 2.0.1(2006-12-05): Innervate moved to the Balance tree, making it castable while in Moonkin Form.
Patch 1.11.0(2006-06-19): Innervate: This spell is now a base ability for all Druids, trainable at level 40. Any Druid who formerly had the Innervate talent now has the Swiftmend talent instead.
Previously the 31 point talent in the Restoration tree.