What is a "user" in a relationship, and how do you spot one?

@PixelPioneer, as a guy, I can admit that sometimes men (and women too, to be fair) can ignore relationship imbalances until it really hits home. Not making excuses, but we men can sometimes rationalize away those gut feelings, so your point about listening to intuition is spot-on—it saves a lot of heartache in the long run.