What are the pros and cons of Pop-up advertising?

It's a difficult subject.

The problem with discussing the pro's and con's of pop-ups is that when you're dealing with pop-ups, the pro's tend to be very visible while many of the con's are typically not so readily apparent.

The big argument against them is that many of the people who don't like them would otherwise have come back to your site in future, but decide not to, because of the pop-up.

You lose some returning traffic, in other words.

All too often people testing them don't allow for this, and assume they're working when they may not be, really.

For example, someone selling a product for $37 can try out an exit pop-up offering the product for $19, attempting to "catch" some of the traffic leaving after people decided not to buy for $37. He can make some sales and imagine "This is all extra money, because these people were leaving my site". It's nonsense, of course, and people who split-test realiably enough, and for long enough, to allow for the loss of returning traffic are typically the ones who abandon these exit pop-ups. (Every client of mine who has tested them realistically and accurately has dropped them).

Some people don't take into account that few customers buy at their first visit to a site, and that most of the $19 sales will be sales to people who would otherwise have returned and paid $37. So these things can really cost you a lot of money, while you imagine that you're actually gaining from them.

Some people genuinely don't appreciate that something can increase their immediate conversion-rate yet still cost them money overall.

Not all pop-ups are "exit pop-ups offering a discount", obviously: this is just one example, but it has some equivalents, too, for other sorts of pop-ups.

The three things to bear in mind about pop-ups are:-

(i) Many marketers use stuff uncritically, without testing it for themselves, because they see so many other people using it and imagine "it 'must' be working" (of course, this is part of the underlying perpetuation-process for many of the urban myths of internet marketing);

(ii) Many marketers do inadequate and/or misleading split-testing because they're not always allowing for all the relevant factors: it's quite easy (especially over the short-term) to imagine you're gaining something when you're actually losing something (in the long run);

(iii) There are already legal issues about pop-ups (especially exit pop-ups), and there'll be more in future, too. Our resident internet lawyer Brian Kindsvater has addressed this issue in other threads on the same subject, and I'm not a lawyer and obviously can't add to what he's said - but it would certainly dissuade me from using them, and that's even without the various other factors mentioned above.