I've been using Yahoo! Ads for about a month now, and I have to say I am very pleased. I can't disclose exact amounts, but I will say that I am earning more money than I have with any other advertiser.
In my experience with advertisers I would rate Yahoo one of the highest, and it's still in beta!
Yahoo! Publisher Network is very similar to Google AdSense. In the sense that both advertisers use contextually targetted ads. In english, this means the text ads displayed are targetted to the content on your website. For example, if your website is about music, the ads displayed will be about musical instruments, music downloads, record stores and so on.
Recently, many publishers have switched to YPN (Yahoo! Publisher Network) from Google AdSense. This means good things for us publishers. Competition will raise prices so that the advertisers can offer the publishers the most income, which means they get more websites to publish their ads and hence more money for themselves. Until now, Google has had little to no real competition, but with YPN coming strong out of the gates, you can expect a good price hike in the days to come.
YPN has all of the standard ad sizes for you to use on your website. From skyscrapers (120x600 & 160x600) to leaderboards (728x90) and boxes (300x250) and more. For most websites, the advertisements will be targetted fairly well. However, in my experience with my graphic art websites, the targetting is horrible. However, this is easily explained by the lack of advertisers since it's still in beta. On my music website, the targetting is dead on accurate.
It isn't all good, especially for you guys who live outside of the USA. Sadly, during beta Yahoo! is only allowing publishers from the United States. So all you guys accross the pond are just going to have to wait. But, if you do live in the US and own/operate a website, I highly recommend you try Yahoo! Publisher Network for yourself.
Heres the link to get started with YPN.
http://publisher.yahoo.com/