I’ve been testing my plugins against WordPress 5 and have had a try of Gutenberg.
My first impression was that it was very poor; I left it for a while and tried again and I’ve changed my opinion to horrific. It may be usable to non-technical people with no experience of WordPress (I’m not one of them so can’t judge that), but for someone like myself, Gutenberg just seems to hide everything away. Also on the hugely negative side, it adds a huge amount of unnecessary comments to a post.
The only positive is that the Classic Editor is still be available as a plugin:
If you install the Classic Editor plugin prior to installing WordPress 5, it should remain the default editor even after the upgrade.
I do have concerns about the Classic Editor being shunted into a plugin and only updated until 2021.