I have a 2011 iMac that's running High Sierra (its last supported release). I upgraded it from its base 4GB to 20GB of RAM.
Since getting High Sierra, everything's been really smooth, but the Sierra -> High Sierra upgrade actually bricked my computer. It was perpetually stuck in the installation phase, bailing after 45 minutes because it couldn't unmount my partition. To Apple's credit though, after I used recovery mode to reinstall the OS, everything worked and my data was intact.
Since getting High Sierra, everything's been really smooth, but the Sierra -> High Sierra upgrade actually bricked my computer. It was perpetually stuck in the installation phase, bailing after 45 minutes because it couldn't unmount my partition. To Apple's credit though, after I used recovery mode to reinstall the OS, everything worked and my data was intact.