Calculating mana returned: see Resto Druid Raiding Guide: Innervate and You.
Innervate's buff is a Magic effect, so it can be dispelled; hence, you should be careful when using it during PvP.
Tips and tactics
Innervate is a great solo tool - it lowers downtime by replacing a drink when cooled, and it can give extra mana during a tough situation.
The mana regeneration this gives you is enormous, especially since the spell's cooldown timer is only 6 minutes. Truly an excellent spell that is beneficial to any mana user, although mana users with high spirit will benefit the most from this spell since its effectiveness depends on the quantity of spirit the most. Typically, classes that will benefit most are, from most to least effective: Priests, Druids, Mages, Shaman, Paladins, Warlocks, and Hunters.
If you lack a large amount of mana regeneration, it may be good to cast Innervate on yourself before you run out of mana, since it takes a moment to give the first few ticks of the spell.
In 5-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.
In raids, Innverate is most effective when used on a Shadowpriest, as the mana gained can be used to deal more damage which with Vampiric Touch, leads to a far greater overall gain of mana than using it on any other person.
