The arrival of Siri is not before time. Just as she can on mobile devices, Apple's personal assistant can understand a wide range of phrases and has a voice recognition engine that works nearly every time. The arrival of tabs in applications gives you another means of working in full-screen in a greater deal of comfort, while Apple's decision to synchronize your Desktop and Documents folders with iCloud feels like a savvy one.
The clear disadvantage here is that you're only going to feel the full benefit of Sierra if you're using multiple Apple devices - particularly ones running iOS 10. Without them, you're still left with a solid update but one that simply can't fulfill its full potential. And it would be nice to see Apple reintroduce some customization with the next release of macOS, even if it's only in the form of a dark mode for apps.
Just like El Capitan, Sierra is a welcome update that brings genuinely useful improvements to the desktop while bringing further interoperability with Apple's mobile devices. There are few shocks and surprises here, and if you were hoping for an entirely new experience with Apple changing to the macOS moniker for the first time, then you may end up disappointed.