its a gimmick. I think its only useful if you are watching something for the first time.
I don't have the S4 but my S3 has a lot of the same features Samsung has thrown in. I've disabled pretty much all of them except for Smart Stay but the amount of times that is triggered in the year I've had the phone is probably less than 20.
It's more of a gimmick to me. I could barely get it to work reliably. I'd actually throw in all those motion/eye features as gimmicks.