The mana regeneration Innervate gives you 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.
When playing as Guardian (tank spec) or Feral in a group, do not forget about this ability. You can use Innervate on party members between pulls to reduce time spent drinking.
It is a great solo tool - it lowers downtime by replacing a drink, or it can give extra mana in a tough situation. It can also help you get ready to continue adventuring again after dying.
In five man instances, it can be a good idea to save Innervate exclusively for the healer when he or she is about to run out of mana. When playing as the healer, do not hesitate to Innervate early and often to reduce downtime between pulls. Just make sure to save it when nearing a boss fight where you think you might need it.
On battle resurrecting a healer or other caster, Innervate enables them to rejoin the fight faster.
/Patch 4.0.1 (12-Oct-2010): Mana regeneration changed to 20% of the casting druid's total mana.
Patch 3.2.0 (04-Aug-2009): 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 (14-Apr-2009): This spell no longer costs mana.
/Patch 2.0.1 (05-Dec-2006): Innervate moved to the Balance tree, making it castable while in Moonkin Form.
Patch 1.11.0 (20-Jun-2006): 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.