Absolutely horrible on my 10 year old 23" Cinema Display and 22" 1680x1050 Eizo. But not all of it, this text I'm writing is fine. But in Mail everything but the message list (system font) window is awful. So yes, reading messages provokes gagging. And no, not a single font works better than Helvetica, except for the system font, but I can't set that for messages. How stupid is that? The system font is also used in iTunes, except for the sidebar when visible (playlists and when an iPhone is connected).
But unfortunately the top menu font is now a washed out grey blur, including drop down menus. Also in Safari – The URLs are ok as are the favorites, but not their drop bar menus.