We are looking for feedback from other entrepreneurs about your experiences.
(1) Paying for clicks you would have had anyway. Many people know precisely which site they're looking for in advance. Googling the site and then clicking that paid AdWords link is simply our lazy way of getting where we were already going. And there are cheaper ways of providing visitors with short cuts.
(2) Paying for non-customer traffic. I don't mean non-converting traffic. Rather, I'm talking about people who visit your site simply in order to promote their own service. You'll be paying for the privilege of hearing more sales pitches. After all, these people find you the same way as prospective consumers.
(3) Providing Google with data about your traffic enables Google to know more about your traffic than you do. Recently Google has begun withholding more and more information. One way or another, that data will be sold back to the same customers who provided Google with the data.
(4) Renting space often leads only to further dependence on rented traffic. The same money can be spent on other services and assets that improve content and visibility or expand one's online footprint in terms of actual holdings.
The answer to that is simple: realistically, you have little or no control over which sites those ads appear on, despite what Google and others tell you. Because of that, you have a very great risk of wasting your money due to "click spam" where the site owners for those display network sites nefariously generate bogus clicks, to earn part of the pay-per-click fee that Google charges. It's a waste of money.