I know what "quick reflexes" in shielding does. But what good will it do for me? How can I effectively use this skill to my benefit?

I've had it on one of my warriors, Your able to dodge the attack entirely and counter attack an enemy, I was never able to master it entirely. It also gave me a second to glance at my compass and see how many red dots there were .

Quick Reflexes allows you to completely dodge almost every power attack thrown at you. This includes all of the troll's attacks, the sabre cat's pounce, the dwemer spere's lunge and many more! If you use block at all, I'd recommend you get it!
It can be tricky to master, I suppose. Just block until the slow time triggers, drop your guard and flank your foe.

Combine it with block runner and you will be untouchable.