If we're talking about Popads.net - yeah, they've been around for a long time and are legit.
If we're talking about pop ads - popup/popunder in general - worked fantastic few years ago and still work, but I'd recommend push notifications instead. I actually started with pop traffic few years ago and switched to push notifications ads and had much greater success with push notifications.
Everything's identical to pop traffic - you run pops and push exactly same way - same optimizing process, scaling, you can use exactly the same landing pages etc. The only difference is that push notifications have a creative/banner in the funnel and THEN when a visitor clicks the banner, he's redirected to landing page.
Anyway, go with push - a much hotter ad format that ROCKS - believe me