Generally speaking people frown upon pop up and pop under ads. However, if you do not have much traffic on your site, you can use pop up and pop under ads to drive traffic on your site. A lot of websites and businesses use pop up and pop under ads. If there was no demand for pop up and pop under ads, there would not be so many advertising companies providing pop up and pop under ads. Unless, you are using adsense, using pop up and pop under ads can be really helpful, especially when you don't have traffic on your site.