Months ago (Oct, Nov. 2014) I’ve played with my freshly installed Yosemite system and some scripts to change the default system font. I’ve tried Helvetica Neue, Input and San Francisco. Here’s how it looked like:
Do you like it? Personally I liked more Input, but after a while I’ve switched back to the Apple’s defaults.
For the record I’ve achieved that result using this GitHub repo, which in the meanwhile has undergone a DCMA takedown notice.