I didn't think the old one was exactly good, but the new one I'm not liking because of the forced tab bullshit. I'd open it in tabs in the browser if I want to open them in tabs Yahoo.
Also, the performance is crappy and its really inconsistent in how it handles images. Sometimes it'll open it in like a slideshow, sometimes it'll make you have to download it.
It was also stupid as they sent me like a billion messages saying how Yahoo Messenger in Mail was being stopped, and yet its there in the new Mail. That's actually a good thing, I just don't know why they made a fuss about discontinuing it when they forced people to switch to the new mail which includes it. They should have used that as a reason for people to switch to the new Mail sooner.