I've been running Yosemite since PB1 on a 27" iMac (2011, non-retina) and I think it's great. As far as readability goes, I think my main complaint early on was regarding the OSX-menu bar and small font sizes used in the Safari UI, but that seems to have gotten better. Or I've just gotten used to it. Not sure. Retina looks much better (according to the pictures), but on a non-retina screen it's not like Yosemite looks worse than Mavericks. It's the same, just different. I'm sure anyone with a high dpi screen will find the low dpi-version of Yosemite to look terrible. But I wouldn't know. Because I don't have a retina screen. So I'm happy. For now. Until that day when I order my 5K iMac and turn all dpi-snobby. A day I look forward to.