I just tried this little sneak/workaround from LifeHacker to see if it would convert my Yahoo Mail account over to the new beta… sure enough, it worked!

I like the look and functionality, but it definitely “feels” slower. Thankfully, I use the Yahoo account primarily for minor registrations (forums, news sites, etc), so it’s not something I have to spend a great deal of time with. Still, if they can get it sped up a bit, it’s a great UI.