I have had a yahoo email account for as long as I can remember. I think it was around 95 or so when I first got my account. I got it because our college email system would not work outside of the classroom. So I figured a web based email would be best, and I think yahoo was the first that I found. When I read this article on how you could checkout the beta of yahoo mail I had to give it a try (Link here).
I really do like the way they have layed out the mail client. They use a lot of AJAX to make the interaction really easy and enjoyable. One aspect that I really like is the tabs. When you send a message or read a message it can generate a tab instead of opening a new window, that is a great feature. They did a good job of allowing interaction with foldering, you can drag and drop your message into a folder. When you do drage the message you can read the subject title and other info while you drag it. An interesting feature is the preview. Just like outlook or any other email client you can preview your message when it is highlighted. I have not see this in a web based email system.
Overall, I think this new beta is great. I use gmail for most email now (forward yahoo), but with this new interface I could be convinced to use yahoo more.