I feel bad for those who are having problems. Really, I do. Two weeks in and Yosemite continues to work buttery smooth here, and on my in-law's MBA. My only gripe is the UI and folder colors, both of which are fixable, as described previously.
Anyone who is having a lot of buggy, unstable behavior should really consider doing a clean install and migrate your user data and apps back from a backup. That's what I did on my iMac. I did a simple install in-place on the MBA, but that is pretty bare-bones with little 3rd-party software on it. Speaking of which, take a very hard look at all your 3rd party software. Make sure anything that loads on startup is Yosemite-compatible. If it isn't, remove it as a trial to see if that was causing issues.