The Yahoo Mail (beta) is huge improvement over the current Yahoo mail. The design is pleasing. Its easy to use. Has some cool features like drop and drag, tabbed email view etc. If you are a user of Outlook Express the transition will be completely painless.
Interestingly enough it didn’t work in the new IE 7.0 beta 🙂
Btw, you have to request access for Yahoo Mail beta and wait for a few months before you get access to it (at least that was my experience).