The main problem I have is that these popups rarely preload the content. This means that if you are visiting the page internationally, you could be staring at a

The main problem I have is that these popups rarely preload the content. This means that if you are visiting the page internationally, you could be staring at a greyed-out page for a good second before the "X" loads. CTRL+F4 is now a reflex to that backdrop. If your page is less important than an advert then I can't imagine me finding it all that important.