I've just upgraded to 10.10 (from 10.9.5) on my system as shown below. Upgrade was smooth through Updates, which included updates to iTunes (which actually updated prior to Yosemite install) & iWorks.
So far, everything is working smoothly with my Mac apps that have not yet updated to a Yosemite specific update. ...So, Mavericks compatible apps appear, as of this writing, to be running Ok. My Mid 2010 MBP seems to be running this OS without any slowdown.
I'll update further if anything key to advise.
Update: Having spent more time poking around my system and apps, I'm confident I'll not run into any immediate issues. ...Time will tell obviously; but, I think 's 3 Gold Masters for 10.10 provided a pretty solid basis for Yosemite's release.
