I never managed to make it work. There's simply not enough mana. 15 points to kill a strong foe? How would that suppose to work? Quickcast is cool, but it wastes, like, 5 mana I suppose? There's no way to lower the amount, you only can get a draw from mana pool in the beginning of fight, but that's capped on 15 mana. One spell that hits for 20-25 will end your supplies with quickcast used. You also need to learn the perk that lets you guard with shield while trying casting, or you get damage.
Mage is also the class with lowest physical stats. Like, you have pretty low Agility and on stat checks you very often get thrashed. We have the Beastmistress Huntress encounter that punishes you for not having 8 Agility with being seriously fucked (currently also puts a Distracted debuff on you). And Majalis actually lowered it even more right now, for what purpose I wonder. Now it's hard to even wear tanky armor, seeing that the default Endurance for Mage is 2.
Also most of spells are useless still.