I'm running it on a late 2014 5K iMac, 2 late 2013 rMBP's and a mid 2011 iMac. I'll upgrade my 2008 Mac mini soon and let you know how it goes on one that old... from what I've seen I'd bet it's smooth.
Every computer I'm running it on is super stable and all havn't rebooted since they were upgraded a couple weeks ago. examples...
Damons-MacBook-Pro:~ CA3LE$ uptime
8:51 up 14 days, 13:23, 4 users, load averages: 1.09 1.22 1.23
Damons-5K-iMac:~ CA3LE$ uptime
8:50 up 12 days, 20:36, 3 users, load averages: 1.73 1.59 1.55
IMO, this upgrade is one of the best I've seen. It addressed some annoyances without being all in your face with changes. Felt like a reboot... accept on the laptops where I REALLY notice speed increases in terms of waking up and getting online. Some browser tabs in Yosemite would take a long time to respond when I'd come back to the computer after leaving for a while... even when the power management was set to an always on state. It would do this especially with Google Analytics and AdSense but google.com and testmy.net would load quickly. It would take up to a few minutes before reloading the tab would have any affect, otherwise it just sits there loading until it timed out. Just on the Macbook's, I never saw that on my iMacs. Using Chrome or Firefox helped but it still would sometimes take 30+ seconds to get on the wifi. After the El Capitan update the first thing I noticed is if it's coming back from sleep it takes only a second to get back on the network. And the tabs that gave me issues load up instantly now. It's saving me time right there... possibly up to 1 frustrating, finger tapping hour a month.