Has anyone ever used popunder traffic for their site and if so what site do you recommend if it was useful?
When advertising with it, it is normally useless, unless your site is very viral/sticky/attractive. If you want to earn money from it, then there are some cpm popups listed here and are reviewed. Personally I prefer not to use popups on my site plus I do not advertise via popups.
I had great experience testing popunder traffic, just be careful from who you buy from. My case study: http://mixedmarketarts.com/2007/11/15/the-good-and-bad-of-buying-traffic/