the best banners exchange you could find is a TopList... each member can enter the list by selecting the category they are related with, and when you apply the toplist script in your site, you can select to display random sites or only sites related to your category.

if it's just an exchange of banners, no need to activate the Top side of things, just a listing can do.. but the logic of the script remains...

benefits of a system like this?! friends, similar interests shared, and better indexation / backlink... if the engine is well written, google and yahoo will index all the list, not only your site, so your reference will raise.