Do you mean the company PopAds or Pop Ads / Pop-unders in general?
I'll comment on the pop-under as an ad format more generically.
Pop-unders as an ad format are generally reviled by users, and because of this, almost no mainstream, well-respected sites uses pop-unders. Browsers have tools that help block pop-unders and from my own experience, they're generally effective, but some pop-unders still persist. One of the few sites where you can see a pop-under live is TheHill (I won't post the link), but that's more of a niche politics site. There are other niche politics site that have these too.
The ads themselves are generally direct-response focused and most of the demand is from the adult / warez / piracy scene. You'll either get a pop-ad from an advertiser like LiveJasmin, or a full-page pop that redirects the popped browser to another site, which is generally chock full of ads. I work with a few vendors that provide traffic from pop-unders and they can be profitable and they are cheaper than native networks. It is technically against IAB standards to have incentivized / redirected traffic without declaring it, but it's one of those things that don't get caught.
If you're trying to build a long-term relationship with users in the non-adult space, pop-unders are a bad play. If you're in the adult space, they're very common.