Well I love the size. The Pixel 4XL is a bit to big for me to operate one handed easily, also bulky in the pocket and the S20 fits much better on both counts.
I'm missing all my saved passwords that I don't know that Google saves for apps and websites.
Overall the UI is fine. It's different but not bad. I don't understand why I need a big header over the "Settings" and "Messages" that takes up the top 40% of the screen, seems like a waste of space. On that note, it didn't take me long to put Android messages and Gboard on it.
It's fast and fluid, battery life is fine so far but I haven't been on it a ton. There's a lot of apps but it's not terrible. Honestly my biggest issue ATM is what launcher to use. The stock one is ok but I'm not crazy about the icons and I didn't see anything I like in the theme store. So I put Nova on it and I'm using the weird gestures where you swipe up from where the buttons normally are. However I'm thinking about going back to the buttons because there's not a big difference and I have always thought Samsung has had the order of the buttons in the right place anyway because I use the back button far more than I use the recents/multitasking button. So it appears for me the only reason to use the stock launcher is for Google's gestures much like with the Pixel launcher.