I'm changing my ranking of this plugin from 4 stars to 1 because it doesn't work on tablets. In fact when it pops up it hogs the screen with the close/delete button to remove the lightbox off the page making it impossible to access.
Also, for some weird reason, although this is called WordPress PopUp, it appears as 'PopOvers' in my administration area. Why?
I recommend PopupAlly as an alternative.