Banner Exchanges or Popunders

Like many people, I am looking for ways to generate free or very, very cheap traffic to some of my sites (without resorting to rolling my own bots). I've seen a few free banner exchanges and wonder what everyone here thinks of those things and if they have any they recommend. I guess the same questions apply for popunder ads.

1) Do you use them?
2) What are any concerns or risks?
3) Who would you recommend and/or use again?

Thank you.

The problem with free banner exchanges is that you pay for what you get: Nothing. The quality is very poor, and making a sale for your products is pretty much non-existent. I suggest you go after keywords that are going to be easy to rank high for in Google, and they can make you potential sales.