For a long time I used Yahoo mail. Now I use Gmail. Why?
GMail is far better than Yahoo mail. Let me explain.
GMail is secure. I can use https access to web interface or use encrypted access via imaps and smtps from any standard e-mail client. And I can use GPG encryption. The best part is no advertizements inducted into my e-mail. It gets better with Google Apps for Domains where I can have the mailbox at my own domain hosted by Google.
Yahoo Mail, on the other hand, sucks quite a lot. No secure imap/smtps support, web interface sucks big time, and injects ads all over the place. Same is the story with their Groups (which I've been using since the beginning). While Gmail has only improved over and over, Yahoo mail is a downhill.
See you on Gmail.
p.s: Windows Mail, Hotmail or whatever crap they call it is not even worth considering. Delivery to their system is a big PITA.