After a couple of abortive tries to use apfs from an upgrade, I converted all SSD partitions to apfs recently, and discovered that:
1. There don't appear to be any downsides, though my own needs don't run to the very technical and I haven't had to do any disk repairs.
2. Copying appears to be faster - I use CCC, though I haven't tried to quantify the speed increase yet.
3. The whole machine runs demonstrably cooler, whereas I was getting regular overheating warnings, they've all stopped
CAVEAT: I haven't converted my HDD drives, the Time Machine drive has to remain HFS+ because TM doesn't work with apfs, and I can't see an advantage to converting my archives HDD, though that may change.
All in all, I haven't seen any downsides to converting SSDD storage to apfs, apart from an increase in boot-time log entries on the screen (a cosmetic issue), which I resolved using a perl script I found on here. Everything hunky-dory really, can't see any pressing reason to go back after holding out for some time.- most of my concerns have proved to be unfounded, and many of those were born of a general disdain for and dislike of most things Apple, though I resist the temptation to go down that path at this time.
