WordPress 5 with Gutenberg is due for release tomorrow on 6th December 2018. While many people, on WordPress.com and other hosted WordPress providers, will not have a say about upgrading, they can mitigate the effects somewhat by installing the Classic Editor:
For those with control over their sites, you can choose to upgrade to WordPress 5 or not (some postings by Matt Mullenweg had suggested that upgrading would happen automatically, but a few posts have been made clarifying that this would not happen).
Much of the community is recommending holding off upgrading until 5.0.1 at the earliest and I strongly agree with this sentiment.
Any upgrade you do, should be on a staging environment first so that you can test to verify that your theme and plugins will all work correctly after the upgrade to WordPress 5.
Whenever you do upgrade your live website, I would recommend having a complete website upgrade (database and files), so that you can roll back if necessary, and test immediately after the upgrade has completed.