I use Yahoo US and it's profitable for me but it's only about 10%-20% of the traffic I see from Google. I have tried Yahoo UK and still do now and again but it's too expensive and every time I try it, it proves to be a total waste of money! I want to try yahoo I little bit more serious, cos my friend get 7200 / month from them. Yahoo is the second only to Google, of course it is worth trying! Most of the webmastes are marketing their website keeping in mind that they only want to rank higher in Google search engine but one should not forget about other search engines like Yahoo, Bing, AOL, etc. I also agree that Google is the best one out there, but still we should also consider other search engines.