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.
You are right about the obtrusion part hobs. But then the whole idea behind this was to add more "targeting" layers make the end solution more relevant to the user trying to exit - so, e.g, you can choose to ONLY implement this solution for your SEM traffic. Ain't that cool for marketers spending a whole lot of money to get users on their websites?