They do work, and are cheap to buy, but you will see far better results with a higher profile ad. Placing your message at or near the top of the publication gives it a much better chance of being seen. Also, many publishers will allow you to include more copy in the higher paying ads. In your own ezine you can take your liberties and write full articles that discuss the merits of your affiliate program. This can be a very effective method, but you need to remember to write it with the readers interests in mind.
Nothing turns people off faster than blatant self serving commercials.
One last method that helps me to focus my promotions effectively is simply to study the site logs and stats pages. Many Affiliate programs will provide you with a stats page. Most of them are only about hits, sign ups and income. This is good information to know, but it doesn't let you know which ads are working the best. That's why I like to create redirect pages on my site.