I'm using Firefox, which has a built in popup blocker. Given that I have a pop up blocker, what makes an advertiser so sure that I want to buy from them that they have to find ways to bypass the blocker? Pop-unders are the spawn of satan, and they are starting to spread beyong Microsoft Internet Explorer. Fortunately, in a very low key way, so far.
Hint to advertisers. If you have to bend over backwards to get your ad seen, chances are you're going to annoy people. Given the choice, I will avoid companies which use popups, and I certainly will never buy from companies which spam.