I have been using Yahoo! Mail since 1995 (same email address) and I have seen it change quite a bit over time. I like it and I even pay the 20 dollars a month to get the premium account. Today I logged in and got an option to try out the Yahoo! Mail Beta:
It is heavily using Ajax capabilities to load emails and populate parts of the application. I have to say that it is rather incredible and feels like you are working with a thick(er) client application than you typical web application. This is a fantastic upgrade. Check it out if you can.