I try for most companies I spend decent $$ with (sorry, no fast food surveys), to fill out the survey if asked to give positive or negative, or both, feedback, because in my business it's important for me. So every time DL asks me to, I oblige, sometimes stating "everything was ok, not bad, not great", and that's that.

However, my flight last week 1 segment was just bad (rude FA, mechanical delay, leaving a 20 minute connection from A to E in ATL), and the next segment was great (FA went above and beyond), and I wanted to point out the difference. Got an email back saying thanks for telling us, and here's 4K miles. I'm assuming it was for the mechanical delay, but I was surprised.