I read fiction about 1/3 of the time and nonfiction 2/3s of the time. When reading non-fiction I often spend idle moments in my day lost in abstract thought about concepts related to the non-fiction book I'm reading. I've noticed when reading novels, I'm far more observant about people in my personal life and their thoughts and motivations. This is especially true when reading works with thoughtful and observant POV characters (especially detective fiction and mystery novels). I think fiction, like music, can serve to frame your mind-state in a certain way.

* It has been useful for manipulating my mood and general mindset. ("My life would be more amusing right now if I felt a bit nervous and unsure of reality. Better go read something by Philip K. Dick." Or "I would be more productive if I were feeling inspired by the grandness of the scientific endeavor. Better go read some Golden Age SF.") * It is useful for understanding what certain situations feel like without going through them yourself, and therefore can help you empathize with people in those situations whose behavior otherwise does not make sense to you. Memoirs and other nonfiction can also do this, but it's easier to find well-written fiction than well-written nonfiction, and for this purpose the writing must be very good.