I have some mixed feelings about the UI - I think parts of it look quite good, I for one like the new Safari and Mail look, especially with floating smaller windows on a big screen. In full screen I think the rounded corners beneath the menu bar makes little sense. The spacing and look of the icons on the right side of the menu bar look a little windows vista or 2010 android to me. I especially don't like the blue to purple look of the menu bar with the Catalina wallpaper at afternoon/sunset. Preview icons in finder look very good to me. Overall it takes some getting used to I guess.
My biggest gripe is performance. Still after 4 days and reboot the system is quite sluggish (on my late 2014 mid-range rMBP). Scrolling in Safari, clicking on the address bar, moving through the icons in the dock, opening apps, everything stutters. Quicklook of PDFs is unusably slow. With just Mail and 3 or 4 tabs open in Safari, I get 6.5GB of Ram used and 50-80% CPU usage.
Also there are still some bugs. One for example is that clicking on a link in mails, WhatsApp, wherever just opens Safaris first tab instead of opening a new tab with the link I just clicked on. Another one is sound output, where I regularly get no sound regardless of media source and output device; after some time or after adding another device like an external monitor, sound output comes back.