This guarantees that I will at least get a specified number of visitors to my site. If my site doesn't sell that's my problem. You purchase the number of times your sites banner will appear on the banner companies pages. This is my least favorite way to buy impressions. You can win or lose with this option if you have a good drawing banner you may get a great response from all the impressions on the flip side you get nothing for your time and money.
Exchange programs are nice.
They don't charge. You place their banner on your page. If there banner is viewed twice your banner will be displayed on their pages once. TSSI Solutions has honestly provides a great number of hits to my site with their free banner exchange. I have not purchased their premier plan however, I might be persuaded in the next 6 months.