Yahoo! Messenger is good only when you want to use the official client of your e-mail. Skype works for all, even for Live/Hotmail users as well as, users, un-related with e-mail, in short, just for conversations.
well i use both, but i feel yahoo is best for text messaging and Skype is best for calls (audio or video).
Yes, I use Skype. Because it is more useful than other. I love Skype to talk with my friend face to face.