Pop-under traffic is cold. In other words, they're not usually on the stage of the buying cycle where they are likely to buy. Nonetheless, you might be able to get them to sign up for a list. Anyhow, despite this fact, a lot of people still say such traffic is junk. However, its big advantage, though, is it's massively low price.
I want to hear your own thoughts about pop under traffic and if you feel it's worth it.