Big Sur can be as close to iOS as one can guess. Everything now has switches, sheets are now just iOS message boxes, progress bars, buttons, …
Not really my cup of tea but the good news:
It runs all my compiled apps just fine without any GUI glitches (well… at first sight).
Did not run Xojo itself though.
The longer I look at the new look of the windows and toolbars, the more I recognize GTK+3, both in the size of the controls and the toolbar design.
The first euphoria has set in and so far I find it optically more pleasant to use the current system UI.
+1
My eyes are getting older. I need more contrast. I can barely see which toolbar item is selected. The same goes for the sidebar of the Finder. And I can’t use “less transparency” because then the menubar applets are white in white.
Did you report it using Apples Feedback App? 
Yup. Already reported. They sure will fix that. But I have made other reports about lack of contrast before and Apple just doesn’t care.
I do not like the UI changes. One example:
Dialog boxes that used to be horizontal, e.g.
[Cancel] [OK]
are now
[OK]
[Cancel]
which breaks 20+ years of muscle memory for me. Sigh.
On the other hand, I have done some preliminary testing of my big complicated app, and so far it’s working pretty well, and looks like a much easier transition than the Mojave -> Catalina transition was.
Each time i drive an Apple Beta, i report as much as possible using the Feedback App. Sometimes it took weeks or months, but i always got Feedbacks from Apple and here and there a fix in an iOS/macOS Update.
Keep on reporting; i have the feeling they are listening. Even if they do not always agree. 
We all know the day will come when macOS and iPadOS will merge into one new System. It is inevitable.
I gave up defending myself and try to concentrate on the positive aspects. 
[quote=493731:@Sascha S]We all know the day will come when macOS and iPadOS will merge into one new System. It is inevitable.
I gave up defending myself and try to concentrate on the positive aspects. ;)[/quote]
True. Big Sur is looking very similar to iOS in many ways now. It’s a clear path to merging both.
I also understand Beatris, some things are now pretty unclear when they are selected or not. Native toolbar buttons (so not the Xojo toolbar buttons) now look very different.
I also dislike the new Folder icons. They look really odd.
Anyhow, I am glad Xojo apps are still working correctly. In fact, it’s the first time it looks like we do not need to make any (major) changes. Phew…
If you have a listbox header then you will either have to recompile with 2020R1 or you need to use NSTableView.
The toolbar also is off by a couple of pixels. There is a gap between the toolbar and the rest of the interface.
This is only the first beta, there’s 3~4 more to go yet! And they still make changes in the GM also, so don’t jinx yourself!
I know. But mostly the first beta are introducing all sort of issues. I don’t think, and hope, it will be the other way around and issues are introduced in the coming betas. 
What I also notice is that the whole OS feels snappier. Also boots faster than my current macOS 10.15
I’ve been hoping that Apple would do something about the poor performance in Catalina. For various reasons I’ve ordered another 16" MacBook Pro, let’s hope Apple BS makes it a nicer experience than the last.
What seems to be faster is checking apps before the first start. Booting only needs to be done when installing new versions of macOS.
I don’t know…
But what i experience is an overall performance boost on my 2017 MBP. And i like the look.
Hmmm… I did found a problem with Big Sur.
The menu isn’t functioning correct. For example I have a Preferences in the menu and it doesn’t show up for some reason.
One icon in the toolbar is smaller than the others.