As of Wednesday Apple have released their latest Operating System High Sierra. The general consensus is hold off.
When you install macOS High Sierra on the built-in solid-state drive (SSD) of a Mac, that drive is automatically converted to APFS. Fusion Drives and hard disk drives (HDDs) aren’t converted. You can’t opt out of the transition to APFS.
Devices formatted as APFS can be read from and written to by other Mac formatted as APFS or Macs formatted as Mac OS Extended, if using macOS High Sierra.
Macs Running APFS can no longer share disks with the AFP — the Apple Filing Protocol. A folder can only be shared using SMB (Server Message Block) or NFS (Network File System). By default, File Sharing (set up in System Preferences > Sharing) is now done using the SMB protocol.
Boot Camp doesn’t read from or write to APFS-formatted volumes, but is compatible with High Sierra.
Reports seem to suggest VMware Fusion will not work in High Sierra.
It's important to note that Apple still hasn't released any documentation on the "proper" way to create a bootable APFS volume. This means should anything go wrong with your installation or disk you’re stuck with the very basic Apple Disk Utility as your only repair tool. No 3rd party disk recovery or repair tools currently exist.
From shirt-pocket the makers of SuperDooper:
Should I Update to High Sierra?
I think I usually tell people to "take it slow" with an entirely new OS version. While there have been "huge win" updates in the past (10.6 is a great example), High Sierra isn't one of those.
Rather, while it may be a good (or even great) update over time, it also has the potential to destabilize things far more than anything that's come before. It's going to be hard to judge its impact until its wide release, and even then, it'll be a while.
If you're a normal user, I would strongly encourage you to not update to High Sierra right away. Let others take the risk. Wait until things calm down and the initial problems, which are inevitable, are fixed. Continue doing what you've been doing before High Sierra: you're not missing anything of significance.
I Insist on Updating Immediately!
Sigh. OK. But don't say you haven't been warned: we don't all float down here.
Before you update, use SuperDuper to make a full backup or two. Check your backup by starting up from it. Then, unplug the backup drive from your Mac and put it somewhere safe.
As I've recommended elsewhere, you should be making multiple backups with other programs as well, including Time machine. So, after you update, continue to use Time Machine to back up. Given it's basically written by the same team as APFS, and Apple knows what is and isn't possible, even when they don't document it externally, this is your best bet to start with.
If you need to roll back to your previous OS, you cannot just use Smart Update. In fact, you also can't use Erase, then copy, because your internal drive has been converted to a storage scheme that won't work with anything earlier than High Sierra.
So, after starting up from your SuperDuper backup, use Disk Utility to erase the drive you want to restore to, by selecting the drive hardware above the volume, not the volume itself. That will roll you back to something that can properly host HFS+ and boot pre-High Sierra releases. You can then restore your SuperDuper! backup normally, using "Restore - all files" and "Erase, then copy" or "Smart Update".