Overcharge is quite useful for both defense and offense.
On defense when monsters are damaging or have already broken through the teams defenses it allows for quick healing of damaged defenses via repairs or upgrades. It also allows the Apprentice to summon new defenses almost instantly to replace whatever was lost.
On offense using Overcharge will allow the Apprentice to use Mana Bomb much more quickly allowing it to be used even when surrounded by monsters who would otherwise interrupt the channeling. It also allows the self heal spell to be used much more quickly, even while in the middle of groups of monsters.
After deactivation, there is a three second cool down before it can be reactivated.
Leveling And Costs
It costs 8 mana to activate Overcharge and 8 mana per second to maintain which will slowly increase in cost the longer it is used. The more points invested in the Overcharge skill, the greater the increase in casting rate and reduction in the ongoing cost.
One of the downsides of Overcharge is that it will consume a small amount of mana every second it is active. This can be particularly bad on maps where you need to conserve mana as it will keep picking up every mana crystal on the floor around you and possibly waste them (if you are overfilling your mana bar). As such it is best to use Overcharge only when it is needed or if the map has an abundance of mana.