I downloaded macOS Sierra because I always like to be the very first monkey in space.
It looks exactly the same as El Capitan, which is good. Other than the notifications, which have changed colour and still don't take on the dark menu theme (seriously Apple, either bother or don't). There's now Siri. I only swear at my computer, so she won't like that. Erm, that's about it. Oh and you can copy and paste from an iDevice. Bless my existential workflow.
It's a bit glitchy, had a colour wheel crash and my iMac went to sleep very slowly early and woke up with an equal lack of alacrity. A restart seems to have cured it. Again it switched off the three finger look-up (which is the most brilliant feature, but Apple insist on turning off with each release). For some reason it asks for my password for shares even though it's in my keychain and populated on-screen and all I have to do is click OK. That maybe my automator action that connects up the NAS though. Never used to ask me.
iTunes still has 'application unresponsive' as its catchphrase. It's like Southern Trains announcing 'we're sorry...'