Most exchange programs don t view the participating web pages to check quality. They seem much more concerned with unique page views. This is where many of your banner impressions may go if you use a free service. These services usually offer commercial banner ad space at a reasonable price, if you don't wish to participate in their exchange program. Mind what you re paying for, though.
Some programs provide a list of participating sites.
It would be wise to take a look at a few of these pages, just to see what the quality is like. Some services require you to place their logo in your banner, as if there isn t a space issue already. Others require that you create your banner in a custom, non-standard size. LinkExchange does both.