[JohnR]

My advice for anyone contemplating upgrading to High Sierra: if you can, hold off.

If you have a simple computing setup with one laptop, you may be OK. If you run multiple machines, various external drives, boot from different versions of the OS, type of thing, then below is a list of things you may find in next week's hair-diminishment program.

• High Sierra automatically upgrades your internal drive to APFS (the new file system). This is well documented, so no surprise there. It's just the implications that follow... (below)
• You cannot reformat an APFS drive back to HFS+ (the old file system). Not from within macOS, that is. You will find instructions on the net that make it seem easy, but they are outdated and wrong. The only way is to boot from an external Linux drive or CD and use hdparm or fdisk to remove all partitions, then reboot from a macOS external drive and reformat the internal SSD to HFS+.
• Sierra or earlier cannot read an AFPS drive. Therefore, combined with the above item, you cannot just revert from High Sierra to Sierra using e.g. a Time Machine backup. Instead, you have to boot Linux and forcibly remove the APFS partitions, boot from an external drive, reformat the internal SSD and then you can reinstall/load from a backup.
• Since Sierra and earlier cannot read APFS drives, if you boot off an external drive that is not High Sierra, you cannot then read anything from the internal drives in the computer!
• Likewise, target disk mode does not work between a High Sierra machine (as the target) and any other version of OSX/macOS.
• High Sierra was unable to format a portable backup drive fresh from the local office supply store. I don't know if it was in FAT or NTFS format, but High Sierra could not reformat it. (Some obscure error I've never seen before). I booted into Sierra from an external drive, reformatted the portable drive with no problem at all, then rebooted into High Sierra to make a backup clone.
• One of my portable backup drives started giving write errors. I had to run First Aid on it, then the errors reoccurred. When I booted in Sierra, ran first aid and then did the large copy, everything was fine. I can't be sure that this was caused by High Sierra and I've now put that drive aside as I'm not sure I trust it any more. But still... coincidence?
• The drive space management in High Sierra is a little .... weird. It does some sort of garbage collection, but it's a little hard to get your head around. For example, put 50GB of data in the trash and empty the trash. The amount of free space now available is slightly less than before! Tomorrow or next week or some time it will (apparently) show as free. In the meantime it shows as "Purgeable." I assume that if you actually needed that space the purging would happen immediately, but I'm not sure. Maybe it's really just fine and I'm just an old computing fossil but I don't like it.
  

In the end, I decided to back out of High Sierra/APFS and am now happily back on Sierra. As you can imagine, this was a very time consuming experience, with the possibility of losing some data, and I strongly don't recommend it.

[JohnR]

Now that I have a Mojave machine, it's unavoidable that I use APFS. And actually, it's fine as long as you just use internal drives. However, my experience is that it's STILL best to avoid using it for external drives.... you would think this not so hard....

But be warned / learn from my mistake. If you have an external SSD and decide to encrypt it, don't do it on a High Sierra or Mojave machine! Why? Because it will automatically be converted to APFS before it starts encrypting. Without notice or warning. The drive that was previously measured at 400-420 MBytes/sec (BlackMagic Speed Test) is now at 40-50 MBytes/sec. It seems the encryption is running at about 5 MBytes/sec, so this (2TB) drive is going to take a couple of weeks to encrypt. Maybe it will be faster when the encryption finishes but I don’t think I want to wait to find out…

The solution is to get a Samsung T5 and format it in HFS+. Then use the Samsung utility to turn on hardware encryption. Result is 400-420 MBytes per second again. As far as the Mac and APFS is concerned, it's unencrypted. There's just an extra step where you have to start the Samsung app to enter the password when you connect the drive.