I just noticed it few days ago, so I was curious to try it. Unfortunately, after 24 hours switched back to the previous version.
Glad yahoo is keeping their older version available. Why? It was way too rough around the edges for prime time.
In other words, you definitely know it's a beta.

I konw that i am just using that because i love to use it . and its very easy to use..

I tried yahoo mail beta, but the main problem i faced was the loading time, some times it fail to response, and when a new mail appeared i tried to open it and the yahoo was unable to open that, so i switch back to the older version, it has less features but when it comes to the speed it is good.