Have no experience with exit pops in adult, but on a mainstream site it increased conversions (=to sale) by about one third when we introduced them - talking about maybe 5 years ago. It was a special offer "just for those leaving".
It lasted just some time: after several months A/B testing showed that there was no significant conversion difference and we removed them because it was bad for brand, customer experience etc. Can't tell if it went worse because of general changing attitudes or other reasons.
Just what i saw, i would not take it as a general rule - as someone pointed out, it depends on a number of side factors.
For a paysite with a brand, or trying to buld one, I would never do it. For TGPs and the like where a fast buck is more important, i would run some testing.
