....We tried Yahoo: It's a complete waste of time and money. Ads were always edited by Yahoo staff (requiring frequent e-mails to clarify the service we were paying for), bids may be changed at random, the interface is a nightmare kludge and there is no way to opt out of Yahoo's 'partner network' (which seems to consist only of spam domain landing pages), so bidding a keyword to the top of the Yahoo search results guarantees a load of expensive visitors from click-farms. I would love to advertise on Yahoo's search result pages and will do so the moment Yahoo Search Marketing is taken over by a competently run organization (Go Microsoft!)....
I am not familiar with the system in USA, but the one in Hong Kong runs like a Mafia. You cannot do this and do that, with entries require real Yahoo! eye-balls approvals.