Thanks Mike. I'm just gun shy these days - Apple seems to be dropping in major changes without any warning, and given their steady decline in software quality, there's a real risk of things choking bad. I plan to run an OSX guest VM and beat the heck out of it before upgrading, so APFS guest support is most critical.
It wouldn't surprise me if they drop some bombs for hosts too, but as you say, there's nothing we can do about that.
I'm waiting for the massive number of 'ios 10.4 bricked my phone and I don't have a backup' posts to hit the forums. This is *not* the time to rely on an iCloud backup! I only wish we had a CCC for iOS :-)