Is it good? I'm willing to give it a shot if I can get more "loyal" users, please share your experiences
it is what it is. When you register, you get both advertiser and publisher accounts. You can spend the balance you've earned as a publisher and transfer that into your advertising side. I'm not sure it actually drives people to browse your site again since it is a pop-up, but you never know... me, I always block ads and pop-ups and its more of an annoyance than anything.
If you know how popunder marketing works and you know how to do it - then yes. Otherwise it won't work. Basically, if you have well designed popunder landing page it usually works fine. If you send traffic to your page directly(like registration page or page with lots of text), it won't do.