Successful (and trouble-free) upgrade of an 8-year-old MacBook Pro from Mac OS X 10.5.8 to 10.10.4
The Mid/Late 2007 MacBook Pro is the oldest MBP that supports Yosemite, but it's running surprisingly well.
It was originally sold with Mac OS X 10.4 Tiger and 10.5 Leopard came out a few months later which I bought on release day at an Apple Retail Store event. Even though I've owned a copy of Mac OS X 10.6 Snow Leopard for quite a while, somehow I'd never managed to upgrade the MBP beyond Leopard
Today, I was nearly stymied to be able to upgrade because you can't go to Yosemite without first being on at least 10.6.8. I thought I'd misplaced my Snow Leopard installer, and I really had no interest in spending $20 for another copy, but after a little searching through some boxes in the garage, I found the installer.
Thus, it's been a FULL afternoon of 10.5.8 > 10.6 > 10.6.8 > 10.10.4. Impressive that I didn't run into a single hitch. All upgrades went smoothly.