I wrote about relatively trouble-free upgrading on a late 2015 iMac 5K a few days ago. Another in-place upgrade with the MacBook Pro Retina also went smoothly.
It has almost been a non-event: in comparison to previous OS releases the transition to macOS Sierra has been the most seamless update in years.
While issues could arise, MPG attributes this at least in part to a relatively minor upgrade that looks to be a major macOS upgrade as a marketing event only. Siri is just an app that could have been tacked on to macOS El Capitan and ditto for the new APFS file system support.
MacOS Sierra is thus a minor release, which is a godsend to users sick and tired of constant feature distruption and new bugs that have characterized previous macOS major releases. By disabling Siri on macOS Sierra, MPG can (gratefully) hardly tell any difference with OS X El Capitan.