I'm actually surprised that Apple let this build of Sierra out as the Public Beta - for me, it has been alpha-level buggy on a late 2013 13" rMBP, 4GB RAM. Most things ARE working, although I'm getting a lot of random slowness. Most things I've been using have **** the bed at some point while using them (can't get Skype to work, Safari starts displaying blank pages and needs to be force-quit, Siri randomly activates on its own and then freezes, Mail and iTunes have quit randomly).
Software Update is showing iTunes 12.5 (which is already installed), and just freezes up if I try to download or cancel.
Yes, beta is buggy, no doubt, and this is not my primary machine. But, boy, I'm quite surprised at how bad this build is.