Juli Clover, writing for MacRumors:
At first glance, San Francisco was a little jarring. After having spent enough time looking at screenshots of the Watch, it's definitely grown on me. I had a similar reaction to the mockups included in this MacRumors article. Honestly, though, I think that says more about how benign Helvetica and Helvetica Neue are. They're such safe choices that—while they don't look bad at all—they don't have the same sort of personality as something like Lucida Grande, Chicago, or San Francisco. Something with personality replacing something that's just... there, will evoke a strong emotional response. Should Apple adopt San Francisco as a system font for iOS and/or OS X, I don't think it'll be long before it becomes engrained in our brains as the Apple font.
It's sleek, like Helvetica and DIN procreated. It stays true to the whimsical and clean lines of iOS 7 and Yosemite, but infuses a romanticized sense of longing for the future where technology and design intersect. Seems like Apple's wheelhouse to me. Hope this rumor pans out. Something tells me we're on to something considering that MacBook keyboard.