I tried the new Yahoo Mail Beta for a while, but I decided to switch back just now.
Short review: it’s nice looking, but feels a tad sluggish. Well, sluggish compared to gmail at least. The biggest problem is that scrollbar in the message list. It’s like promising that you can scroll through all your messages, but when you try to scroll to a far away part of the list, you just get a lot of “Loading… " text. I doubt that scales very well, especially as you get to the thousands in your number of messages.
Google is being smart in that Gmail will only display at most 100 messages at a time. With their compact layout, that’s still a lot more than most other email webapps/clients.