I don't get it. Yosemite and all of the betas have been rock solid for me on a non-trivial, 16GB mid-2013 MBP setup (external monitor, multi-OSvirtual machines, wired and wireless networking, multiple memory intensive graphics design, photo and video editing apps, Xcode, even Office for Mac and Office for Windows).
I read the comments before updating and so far I have *upgraded* to every beta without issue. For me, Yosemite remains the best OS I've used -- though I believe Windows 10 is shaping up to be an awesome contender.
Net-net: I sympathize with those who are plagued by Yosemite issues, but the OS has been a stellar performer for me .. and I'm scratching my head for an explanation why some are experiencing the polar opposite.