Apple's unified system font for OS X, iOS and watchOS is here. For someone with a degree of OCD, having the same type face on all my devices is more then welcome. This is what amazes me about Apple; the amount of work they put in every single detail for a feature like font and unify it across the eco system. Just Brilliant.
San Francisco fonts are available for download on Apple's website (Beta) and don't forget to watch this session on San Francisco fonts from WWDC 2015.