And by that I mean having a broadsword in one hand and a magic spell in the other.

(Yes, that was a Dan bull reference )

I've heard some people say that its underpowered.

There are very few methods of fighting in Skyrim that are underpowered. You can run around with a one-handed weapon and anything in your other hand and you will still kill enemies efficiently. If you make use of your hotkeys, a spellsword with a one-hander mainhand, and 3-4 spells being rotated in the offhand is a very powerful way to fight.

On my evil sorcerer character, I basically fight with a sword always swinging, and my other hand either summoning a minion, healing myself, shooting fireballs, using a ward to absorb spells, turning invisible, or even casting magelight. Its all gravy.

I don't prefer it. I prefer a shield in one hand and then either a spell or weapon in the other.

Its pretty darn good. The versatility of have a melee and ranged weapon on you simultaneously is great. Plus its fun as [censored] to blow things up with fireballs and lightning bolts. Doesn't have to be destruction that you use though.