I've been playing something close to pure destruction mage on adept level. It's pretty good. I'm not doing any enchanting, just using the stuff I find. I use alchemy and it's great to have, but I rarely need to use potions. It's been pretty smooth sailing after I got to level 40 in destruction and got impact perk.

My current gear is: Archmage's Robes, Morokei, Amulet of Savos Aren, 15% Destruction Ring, and 40% Resist Shock boots. Magicka 390, Health 150, Stamina 100 with gear. Magicka Regen is +200%. Atronach Stone.

I'm level 20 now with 64 in destruction and 51 in alchemy. I use Fireball mostly which does 60 damage in 15 foot radius area. I can cast 10 Fireballs before I run out of magicka. That's 600 AoE damage and kills off most lower level enemies. I usually sneak to get in position and use Fireballs from as far away as possible, and run away and hide to recharge magicka if there are survivors. If running away is not possible, I switch to a lower power Firebolt and try to stun lock using dual cast. If I have no magicka left, I switch to Flames for a while until magicka regenerates enough.

This is a good point. I didn't use fireball or any other AoE spells at all when I had Lydia as my follower in my previous playthrough. I'm doing solo destruction mage now and I'm using fireball almost exclusively.

Not really broken, it just doesn't scale at all which makes it fairly ineffective as you level up. This is particularly true of lower level spells which become useless at higher levels. The result is that a destruction Mage ends up with a meager handful of spells that are worth using. Even those aren't as effective as they should be.