And so far, so good. A few little glitches here and there (all of which I've dutifully reported back to Apple) but overall Yosemite has been much more stable than I'd anticipated it would be.
The biggest problem I've encountered has been the spontaneous dropping of the connection to my Bluetooth mouse and the subsequent refusal to reconnect, requiring a complete reboot. I'm not a hundred percent sure this wasn't a problem with the mouse itself, because it the poor thing was four years old and this behavior had been happening occasionally under Mavericks, but to rule out the mouse itself we bought a new one and the problem has disappeared for the most part.
Other issues have been mostly graphics related: items not aligning properly, text overshooting other elements in windows or being cut off, inconsistencies in what is translucent versus what is not from application to application; things that don't prevent me from productively using the OS, but definitely need to be addressed before this is shipped.
In short, I'm still very impressed what Apple has done here.