I use banner exchanges, but in addition to my other main advertising. I have banner ads at the top of the page that I sell to people, Google ads in the right column, and then at the bottom of my websites I have my banner exchanges. I use ad rotation software, so I signed up for a bunch of banner exchanges, and they rotate through the bottom of all my sites. Since I have over 270 websites, having those banner exchange ads on the bottom of my sites have generated a small but good amount of traffic that I would not get otherwise. So I would recommend trying to work it in with your other ad campaigns but not make it your primary ad focus. One site you can see how I layout my ads is here: http://www.FindYourHope.com
I have reviewed some of them on my website. The two that I recommend are linkuddies (formerly smartclicks) and ExchangeAd. Please see the following page for more info: