Ive had all kinds of problems with Overture / Yahoo Search Marketing, especially with them over charging my credit card. Then trying to get them to reverse the charges is a herculian effort, the customer service is not very good. It seems like many other people agree with me. Heather posts some rather damning criticism of YSM.
Ive talked with many other people who have basically cut back on Yahoo/Overture and put that money into Google adwords, thats a wise choice in my opinion. Not only does Google adwords produce more focused and qualified traffic, but you have more control, its easier to manage and Ive never had a problem with customer support.