I find pop up to be my least favorite. They work but you need a very eye catching page to bring them too. I find because they hit your page every time unlike a banner ad you have to get a banner click for them to visit, you do pay a bit more.  They are worth a try, a small amount but watch them amount of user you get pre dollar, if you are getting what you wanted I'd continue. If not I would look at different ads. Must if not all ad companies offer pop up so some people most be doing well with them.