Just clean-installed it on a separate partition on my early '09 iMac 24" and so far I'm much more pleased with it than I was when I first installed 10.9 same time last year.
Still have the wake-from-sleep bug that shows the wrong time on log-in screen. But I had that bug in 10.9 so I can't complain too much.
After inputting all my settings, iCloud, etc.... I had one issue where my Notes app was showing duplicates for all notes. Found that it was seeing two iCloud accounts, the same one twice. Fixed it by signing out and deleting all iCloud off the mac then re-enable. Seems to have fixed it.
Performance is surprisingly smooth on this old mac and safari scrolling is buttery smooth, more so than 10.9.
Will be testing this for a week before full migration, in the meantime I'll keep the 10.9 partition intact but pretty happy with it so far.