I'm running OSX Yosemite on a 2006 Mac Pro 1,1. It wasn't too difficult once I had the correct boot.efi, and it works flawlessly. With an SSD, a minor CPU upgrade, and a recent video card it's nearly as fast as a 2013 Macbook Pro. I'm thinking of switching it over to Linux though, I'm no longer happy with the direction in which Apple is headed, and I do nearly everything in the terminal and browser anyway.
If you do switch, look for a KDE-based distribution like Kubuntu or Antergos. you'll thank me later.
I should give KDE another chance. Last time I tried it was like a decade ago.