It's an amazing framework. Sure Flutter and others came first and already did parts of what SwiftUI+Combine do, but being able to take an app from design mockups -> interactive prototype -> final version right in the same editor and using the same language for UI, logic and model (with native APIs and look-and-feel) is something I've fantasized about since forever.
There are many rough edges (just like Swift itself which took like 3 years to become good enough for "serious" use), you still need AppKit/UIKit for some things, and Apple's developer documentation remains appalling as ever, but I was able to turn a bunch of ideas into usable apps in a matter of hours, that traditional frameworks would have taken me weeks to do.
I hadn't even planned or expected to make "proper" apps with it, I was just playing around, iteratively adding to mockups, and boom, there was an actual app on device.
Catalina itself however was the buggiest macOS beta for me. I permanently lost random files on iCloud Drive, CPU and network usage was always high, and I had to use a temporary local user account that was not tied to iCloud to avoid messing things up.
And it's annoying that they still haven't fixed some minor bugs I reported since the Mojave beta, like unreadable white text on brightly colored tags in Dark Mode and Books jumping around between desktops on its own (the kind of stuff that would have infuriated Steve Jobs I assume but is deemed unimportant now.)