Banner exchanges promise Webmasters they'll bring your site new visitors. If that were true everyone would be joining many banner exchanges, but fewer than 5 per cent of the people who see your banners will actually click on them. In order to benefit from a banner exchange your Web site must already be generating traffic. The exchanges won't bring in people for you until they display your banners, and they won't display your banners until you get people to visit your Web site and view the exchanges' other banners. If you can find a specialized banner exchange that focuses on your Web site's subject area, you'll be more likely to realize a benefit from joining that exchange once you start getting regular traffic.
People are more likely to click on a specialized banner exchange than on a general exchange.
General banner exchanges are beneficial to sites with a lot of traffic. You should target your banner displays as much as possible but get your site name and URL out there where people will see it. The object is get people to remember who you are. One method of increasing traffic to your Web site is to provide free content to other sites. That's easier said than done.