Ethan Zuckerman puts it better than we ever could in this Atlantic article where he apologizes for creating the popup ad, among other mea culpas.
As a rule, the ads that are worth the most money are those that appear when you’re ready to make a purchase—the ads that appear on Google when you’re searching for a new car or for someone to repair your roof can be sold for dollars per click because advertisers know you’re already interested in the services they are offering and that you’re likely to make an expensive purchase. But most online advertising doesn’t follow your interest; it competes for your attention. It’s a barrier you have to overcome (minimizing windows, clicking it out of the way, ignoring it) to get to the article or interaction you want.
So should you buy an online ad for your small business? Sure, as long as the ad you are putting up is aimed at people who are ready to make a purchase right away. For most people browsing the internet they are not in that position, so the likelihood that you should buy an ad is very small. You also will need a budget of thousands of dollars per month to make a difference, even if you do decide to advertise. The investment is much better spent towards building a high quality business model as well as hiring the best people you can to help your business grow. Don't support Google's payroll, support your own.
