Yahoo Search Markrting is the best marketing system in the world. Right from the beginning Yahoo has served as the realistic and affordable to the users woldwide.With this motive in mind. yahoo has an entry and out source in marketing
As someone has said earlier, it depends on what you are marketing. I have found yahoo to be very liberal in applying impressions. Which of course gives you a better opportunity to get clicks. Yahoo is my favorite. I have also used Google. Much harder to get impressions and of course clicks. Just my observations...