It's an incredibly subtle feature. I think too subtle; I'd rather see it have more of a pronounced effect.
Basically the higher your DETAIL LVL is set, the more effect it'll have. If you have DETAIL LVL set to +7, you may see a bit of softening in skin tones. But if you have DETAIL LVL set to -3 or less, I doubt you'd see any effect at all, even under a microscope.
So it doesn't hurt to leave it on; normally you'd use it when the subject you're shooting has some sort of skin issue (such as acne, or an elderly person's wrinkles, etc).
