John Gruber, replying to this article by Thomas Phinney, writes:

Here I disagree with Phinney. I don’t think Apple has ever promoted Helvetica Neue as being more legible than, say, Lucida Grande. Apple has moved to Helvetica because it’s more attractive, and, with modern display resolutions (especially retina displays), Helvetica is legible enough. One may fairly argue that legibility should always trump aesthetics — but one could argue thus for all font choices, not just UI fonts.

I think that it really depends on the specific purpose for using a particular font. Sometimes æsthetics may be more important than sheer legibility (the first example coming to mind is music album artwork), and let’s leave aside for the moment the fact that there are many fonts which are both legible and æsthetically pleasing. But when it comes to user interfaces, I believe that legibility should really always have precedence over æsthetics in the design process. That doesn’t mean that a UI font can’t also be nice to look at, but a UI font is, above all, something you use, not something you show off. A system font is a UI element (perhaps even the most important UI element) whose main job is to help users with their work. It’s a means to an end. Icons may afford to be just pretty. Not system fonts. System fonts should be, first and foremost, clear to read in the most diverse scenarios.^[1]

I don’t know how exactly Yosemite’s system font looks in the latest OS X beta, but the people I’ve heard so far all agree that it’s harder to read on non-Retina displays, and that’s unfortunate. At the moment, only MacBook Pros come with Retina displays. Even if Apple introduces Retina MacBook Airs and Retina iMacs later this year or in early 2015, OS X Yosemite is going to be compatible with a lot of non-Retina Macs. And even if Apple introduces an external Retina monitor, you can’t expect all those buying Mac minis and Mac Pros to choose that over more affordable, non-Retina solutions.

If Yosemite’s system font is clearer to read on Retina displays than non-Retina displays, that in itself is enough to demonstrate how a step back it is from Lucida Grande, which is very legible on both types of displays. Taking a design choice that from the start is going to make your operating system look worse on a lot of compatible machines is not, in my opinion, also a good typographic choice. OS X is not iOS. Helvetica Neue, while not a optimal candidate for any system font, is tolerable enough on iOS 7 because at least the majority of devices supporting iOS 7 has a Retina display. 

I never liked to express opinions in the form of “Apple should do this and that,” but from a typographical standpoint, I think a better course of action would have been to slightly tweak the already more readable Lucida Grande to look ‘fresher’ rather than trying to adapt a more difficult font such as Helvetica Neue to serve a purpose for which it’s not really suitable by default.

(For other personal objections on Helvetica Neue, see also Helvetica Neue as system font is a bad idea.)