After spending an entire day putting Yosemite through its paces - testing ALL my software, plug-ins, etc. I'm happy to say...
Yosemite passes with flying colors for me! And I'm super OCD about my systems. The transition was seamless, and all my DAWs, sessions, and plug-ins run as good or better than Mavericks.
That being said, I made sure ALL my software and plugs were updated to the latest version prior to performing the Yosemite upgrade.
Here's a few details if it helps anyone which appear to be working perfectly :
Syncrosoft & iLok drivers
UA's Apollo driver software (latest version 7.10.x) works flawlessly
Cubase 7.5.30 64-bit (have it pre-installed on Mavericks before the Yosemite upgrade. Otherwise you will need Steinberg's Yosemite install tool - but some users are reporting issues even with the tool.)
Pro Tools 11.2.1 64-bit
Adobe Audition CC 2014
Kontakt 4.5.1 64-bit (and I assume most, if not, all other Native Instruments plug-ins, though I read on other forums that Maschine and Guitar Rig are having problems)
Spectrasonics (Omnisphere, Stylus RMX) 64-bit
Pianoteq 5.1.0 64-bit
Waves - the plug-ins run, however be aware some forums report the WLC app currently has a bug that prevents you from authorizing new purchases or transferring licenses to/from the Waves cloud
I ran into tricky in my travels:
Avid's legacy hardware drivers are touchy. I tried an MBox 2 mini and it was not recognized by the system right away. I've posted instructions for getting it to work on Avid's DUC over here .