I use it a lot, almost every fight depending on the enemies. Its main use after it got nerfed is completely stopping enemy Archers/Rogue. Here's a simple combo I use to destroy ranged mobs in a few seconds on Nightmare.
Fade Step Lv.2 (Into a mob of enemies)
Disruption Field
Fire Glyph Lv.2 (This will automatically hit since they can barely move)
Fade Cloak Lv.2 (You should have enough for this if you're standing by at least 3 enemies)
If all 3 spells crit, you're looking at around 12-14K or more damage depending on your gear and stats. If you do this quick enough you wont even get hit. I might make a short 15 second clip of this if you need. Also, my Disruption Field lasts a bit longer because I have Varric in my party so that also helps. And lastly don't waste a point on upgrading Disruption Field since it's rather useless because you're going to want to hit that enemy while they're in the field.
What Bioware should have done is make Disruption Field deal 150% Spirit damage per second to enemies inside the field while it's active. That would've been much better than that silly 99% slowdown.
And instead of that silly useless Focus heal, they could've made it focus on Spirit Blade. Something simple yet elegant. For example: Sword Of The Fade - Extends Spirit Blades range and damage while active, and every third hit deals Spirit DoT damage. Or they could've made you summon another Spirit Blade so that you can dual wield them for 10-15-20 seconds while giving them increased damage and speed. Anything besides that heal would've been better.