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.
From everything I’ve heard, the transition to macOS High Sierra was easy and rock solid. But, even with that said, the transition of a file system makes it especially critical that you have a reliable, up-to-date backup of your Mac before you give High Sierra a try.
If you are planning a move to High Sierra, read the knowledge base post for specifics. It’s not long.