It works, but there are some bugs.
The Find My Friends complication/app has disappeared from the Watch. In iOS 13, Find My Friends was replaced by "Find My," but there's no "Find My" in watchOS 5.
I'm also having trouble with complications for third-party apps. I can no longer add them to watch faces, though complications previously added to faces keep working. And if you try to edit complications and switch away from a third party complication, it can't be switched back in.
Activity and Workout tracking is working fine, so I'm ok with it until the new watchOS gets officially released.