I think this is a nice alternative to the hideous window.onbeforeunload popups. However, the popup animation is very slow, I would say just flip the css display to block for an instant pop. And because the popups are at the bottom, they're easily missed.
This might be cool from a js side, but isnt it really scummy to shoot out a pop up when someone is about to leave?<p>Maybe its just a pop-under or something to that effect... but gross.