I just updated to Yosemite, but I had a ton of problems along the way. Most of them were my own stupid fault though.
I did a format and clean install of Yosemite, however I had no sound. Stupidly, I just used my Mavericks MultiBeast settings to use for Yosemite, and I forgot to uncheck USB 3.0. So a reboot and no working USB ports (and thus no keyboard/mouse) later, I had to reinstall the whole thing over again. So I did, again no sound but everything seemed to be working okay. Had some freezes and kernel panics, but nothing extreme. Still no sound though. Since it all seemed stable, I migrated my backup to Yosemite.
I eventually downloaded MB 6.1.0 and used ALC898: reboot, still no sound, but at least I had some options in sound settings now. I read in a thread somewhere that it's possible to change output in the MIDI setup, so I messed around in it and found out that Built-in Output worked for me.
So finally after 1,5 days of messing around, everything so far is working properly. Lesson learned for next time: don't be so hastily when upgrading to a new OS.
Edit:
Aaand sound is gone again, again no options at sound output. I'll see if I can fix it tomorrow, hopefully for good.