Interestingly they've built two different versions of the typeface: Compact (for watchOS) and UI (for OS X/iOS). Compact is far closer to DIN or Roboto (with flattened vertical strokes on glyphs like the uppercase C, D and G), whereas UI is far more rounded, and looks a lot like Helvetica for some glyphs.
I was concerned based on what I'd seen of the typography of the Apple Watch that it'd be too cold and characterless (I really dislike Roboto on Android) to use across OS X and iOS, but by creating this separate UI variant they're able to make it feel like a gradual transition from Helvetica Neue. It's so similar, in fact, that I'd not be surprised if we end up seeing more font quizzes asking: ‘Arial or Helvetica …or San Francisco?’
I was concerned based on what I'd seen of the typography of the Apple Watch that it'd be too cold and characterless (I really dislike Roboto on Android) to use across OS X and iOS, but by creating this separate UI variant they're able to make it feel like a gradual transition from Helvetica Neue. It's so similar, in fact, that I'd not be surprised if we end up seeing more font quizzes asking: ‘Arial or Helvetica …or San Francisco?’