I have been seeingg posts about destruction magic and how it is useless and I have always favored that school of magic over all others (besides restoration) so I made an Altmer pure mage who uses only destruction magic. I only focused on leveling up magic and I encahnted all of my items to boost my magicka so I now have about 528 magicka (I am level 21 now). I kill dragons in a few firebolts, I killed Morkei with like 6 fireballs, and I can just obliterate anything while barely losing magicka. Honestly I dont know why you guys are saying its incredibly weak. Damage also depends on what enemy you are fighting and their resistances.
I ONLY focused on upgrading magicka and I have enchantments that boost my magicka. So for those talking about running out of " mana" or "ammo", I do not have that problem.