Well, I have studied typography, and Helvetica Neue is one of my favourite fonts.
Been running Yosemite for about 24 hours now... and the new fonts look strange. Helvetica is a great font family, but it doesn't seem like a system font. In my mind, a system font should be one that you wouldn't really use anywhere else. Sure, you could write a Word document in Lucide Grande - but you probably wouldn't. Or, going back into dark ages, you wouldn't set anything in Chicago either.
But I use Helvetica Neue quite a lot... and to have it as the system font seems to be more just a fashion choice than a practical one. It is perfectly legible on my retina display, but that doesn't mean I really want it all over the OS.
Overall I do like the look of Yosemite, but more than any other iteration of OS X, this one really seems to be following a design TREND rather than any practical functionality. Which is to be expected, now they have a designer designer in charge of the UI, rather than a UI designer.
Which also means I would expect another UI revamp in say 10.12 in 2016, once flatness becomes passe.