Criticism of the Yosemite UI, in particular of the UI font, has been sufficiently widespread that I would hope that someone is paying attention. The sporadic blurring is really hard to fathom.
However, now that I have installed cDock to make the dock less disturbing, and am using Fira Sans as my system font, I am finding Yosemite more palatable. I have also set up one test system with Lucida Grande as the system font. The difference was actually rather shocking: Lucida Grande is so much more legible and so much "cleaner" than Helvetica Neue.
This article, in which Jens Kutílek is interviewed, covers some of the problems pretty well: http://www.howdesign.com/web-design-resources-technology/****-helvetica-yosemite -backlash/
Neue Helvetica is just a bad choice for user interfaces. Its spacing is very tight, which gives words a very “designed” look, but it’s bad for word and letter recognition. Its letter forms are closed, for example the lower right terminal of the “e.” It goes all the way up until the end reaches the horizontal, closing up the interior form. The outer shape gets therefore quite similar to “c” and “o” at least on normal resolution screens.
On Retina displays the higher resolution helps, maybe that’s why on the iPhone, Helvetica doesn’t bother me that much. Requirements of UI fonts are similar to those of signage typefaces. In that field, Adrian Frutiger set the standards in the 1970s when he developed a signage alphabet for the French airport Roissy. Some people expected that he would use his Univers family, but he knew that its closed, static forms would make a bad signage typeface. The Roissy typeface later evolved into the Frutiger typeface family. It’s still one of the most widely used signage typefaces. Microsoft uses a very similar font for their Windows systems, Segoe UI. The open letter forms and generous spacing maintain the characteristic shapes of the letters even when viewed under bad conditions (low light, low resolutions) and from far away. Another point are the numbers, which are also quite important in user interfaces. The Helvetica numbers are very hard to recognize at a glance, also due to the closed forms.
Kutílek is a Font Engineer at Monotype in Berlin.
When you use H. Neue for print, you have to be careful with spacing at smaller sizes, and print tends to be clearer than what you see on screen, unless you are printing on cheap toilet paper.