If you're good at negging, it works really well. If you're good at it, it doesn't necessarily even look like a backhanded compliment. It's just taking what the person thinks is so great about themselves, and pulling the rug out a little. There's a YouTube video of a talk show segment with Jessica Alba, and the 2nd guest was a famous Pickup Artist who was pushing his book. Jessica challenged him to try the pick up tricks on her. He said something like, "Well, okay, but before I do this, I need to know a little bit about you and why you're valuable to pursue. Here in Hollywood, everyone is beautiful, so you can't tell me that the thing you bring to the table is that you're beautiful. Tell me some other reason."

AND SHE DID. AT LENGTH.

(For those who don't get it, his opening was negging. He was devaluing her beauty, and forcing her to try to win him over by talking to him about other things in her life/character that were valuable to a relationship. I think she tried to convince him that she was very loyal. That was her thing. In the end she eventually seemed unimpressed with his pickup stuff, but without realizing that he had used it on her in the opening and it had successfully caused her to begin talking about herself and why he should want her. He essentially went from being told 'try to win over Jessica Alba' and flipped it so she was trying to win over him. Nobody in the audience got it. But I was just like, HOLY SHIT, THIS GUY.)

Maybe somebody could find the video and post a link to it?