I upgraded. First thing I noticed was that Discord and Slack needs to update their code to re-hide the title bar.
Secondly, I spent about half an hour converting a HFS+ Case-Sensitive drive to APFS. The reason why it took so long was the fact that I couldn't convert it while the OS was running, so I had to figure out how to reboot into recovery mode (I rarely need it

), got it converted with no problem, then updated /etc/fstab changing
hfs to
apfs and we were good to go.
Thirdly, holy hell they were not kidding when they said folder sizes would be instant. My aforementioned mounted partition has about eleventy bajillion small PHP files, and the entire folder was sized pretty much as soon as I ticked the button. Seriously impressive work.
I also launched all of my commonly used apps, and they all started up without issue. Haven't actually used them yet, but they all seem to work fine.
Overall, I'm plzd with this upgrade