APFS conversion is mandatory for SSD-equipped Macs

Key quote:

"When you upgrade to macOS High Sierra, systems with all flash storage configurations are converted automatically. Systems with hard disk drives (HDD) and Fusion drives won’t be converted to APFS. You can’t opt-out of the transition to APFS."

I personally suspect there will be a hidden option to skip the automatic conversion (which I plan to use), but still--this is an aggressive rollout. Very impressive, but also a bit scary.

APFS is designed to work more effectively on flash based storage.