@nbozukov, This was the first time I heard of pop-unders ads. I guess I am not so much in the advertising business. Wikipedia states that because they go under the active browser window when you finally spot them, it can be harder to determine which site opened them (especially if you are like and use lots of browser tabs at the same time) so it could be a nice trick for your site to do advertising sneakily without fully bearing the negative effects on people's attitude towards ads.

Well? many well-trafficked online media sites have pop-up ads. For instance: CNN.com; ABCNews.com; washingtonpost.com; the website of The New York Times and etc.

Nevertheless, you are right that pop-up ads are much more obstructive than the banner ads. Probably that is why they are much more successful hence expensive. Advertisers pay almost 5 times more for a pop-up ad than for a banner one.

What do you think about the pop-under ads? Actually, I think they are much less intrusive and successful than the pop-up ads.

@nbozukov, Pop-ups are more or less the same as the other forms of internet ads such as banners or google ads but are more obtrusive since they are more difficult to ignore. I am almost professional in ignoring internet ads, I rarely even notice banners or short text ads and even more rarely click on them. For the banners it takes a little more effort for the user to get round it - you need to point your mouse to the popup close icon and click it. In the meantime your attention is directed to the popup content unintentionally even though for only a moment.
I am surprised of the results of the survey you mentioned about but I am ready to agree with them. Probably the the bigger part of the Internet users are not so much annoyed with pop-up ads after all.
Nevertheless I can't think of any big web site which shows pop-up ads. All the major social networks and big news websites don't show them. (Correct me if I am wrong here).
Probably pop-ups bring higher capitalization out of the existing user mass, but hinder the user mass from growing?

Furthermore, this type of advertising is mainly used to increase the web traffic or collect email addresses

A recent Dynamic Logic study shows that Internet users are actually more tolerant of pop-ups than previously thought. Results of the survey showed 72 percent of U.S. Web users accept limited use of pop-ups, and 47 percent agree as many as two to six ads per hour are "appropriate" to support free content. So it appears that some users are tolerant of pop-up ads.

Don’t you think that pop-up advertisement is essentially the same advertisement, we see everywhere around us, but in a different order to catch our attenetion?

I am truly irritated with pop-up advertisements and try to find my way around them whenever I can. Pop-up blockers are preferred, though when certain websites still manage to provide irritating pop-ups my traffic to those websites will be decreased. Often those ads that do pop-up are obnoxious, about for instance online poker games or a claim you were a winner of an iPod. The design of these pop-ups does not appeal to me and without being able to provide sources that back this up I do think they target a certain audience, namely the type of audience that does get triggered to play online poker and those that believe that they truly have a chance of being the 1000th selected winner for an iPod. I do see the use in a pop-up advertisement as apparently some people are interested in the services offered. However, companies should be very careful in deciding which audiences they want to attract, because many clients, myself included, will be annoyed by the pop-ups. Advertisements also do not seem to be personalized, as services offered in the pop-ups I receive do not appeal to me. I am a mac users, but I for instance do not believe I can win an iPad through such pop-up window advertisements.