I upgraded one of my systems from El Capitan to Sierra this morning. I had a moment of panic when Sierra didn't show up on the Purchased tab of the Mac App Store, although I did in fact have a working copy of "Install macOS Sierra" in my Applications folder.
I tend to like these refinement updates, like Snow Leopard and Mountain Lion, but I have to say that APFS gives me pause. If Apple doesn't have enough confidence in it to deploy on anything other than SSDs, then it doesn't sound like it's fully baked.
Sierra was the first smooth upgrade I've experienced on Apple. Every previous version was wrought with core items breaking that couldn't be fixed, and required restoring backups. I find it's not the new features that mess things up, but Apple's desire to make existing functionality break.
I tend to like these refinement updates, like Snow Leopard and Mountain Lion, but I have to say that APFS gives me pause. If Apple doesn't have enough confidence in it to deploy on anything other than SSDs, then it doesn't sound like it's fully baked.