The way I see it, it works like so:
Let's say there's 2,000 people in the exchange.
Along comes Bob who joins the exchange today and gets his 1,000 sign up credits.
Bob's site only gets about 100 hits a day. Let's say everyone else in the exchange gets about the same. So let's say within the first hour or two, everyone else in the exchange gets 10 to 20 hits, and on half of the sites, 1 of those hits gets Bob's banner exposed once. Bob has just used up 1,000 credits. If he gets 100 hits a day, he's earning 100 credits a day.
Maybe I'm wrong, but if I am there's tons of banner exchanges that are all wrong as well I guess.
http://www.bannerpad.com/ 1:1 plus 250 free credits
http://www.mewho.net/bp/ 1:1 plus 2,000 credits (not free to join)
http://masterbanners.com/index.shtml 1:1 plus 5,000 credits
http://poker-play-zone.com/Banner-Exchange.html 1:1 plus 10,000 free credits.
Thanks,
Bill