I finally was able to use the Power Drainer successfully a few times last night. It's not useful in open areas since people can scatter to get out of it's range, but when players are boxed into a tight area it can be devastating.
I deployed it once in Snowbound as I was being chased inside one of the bases to use the shield door as cover - I was outnumbered. I dropped it just around the corner so they wouldn't see it until they were inside, and looped around the cover just inside the base. There's a transparent wall there so I stood where when could see me - as bait. It worked, and they tried to come around the cover in a pair, right through the Power Drainer range. I circled around the far side of cover to keep it between us and they realized what had happened. They backpedaled but by the time they got out in the open their shields were down and it took a quick spray of AR fire to drop them both.
Stuff like that really shakes up the combat and is incredibly satisfying when used successfully. Of course my kill:death ratio reveals how often I've had it used against me successfully as well.
Another incident: I charged a guy inside a bubble shield, and he backed out. I thought it would be a standoff when I was blown to smithereens. He'd dropped a proximity mine inside the shield. It killed me, and the shield protected him from the blast. Bastard.