This morning the in place upgrade went smoothly (I joined macos ecosystem at high sierra, since then I always upgraded, and one time transfered from mb 15 to mb 16. This experience is surreal compared to windows word.)
Big sur seems faster than Cataline, and I didn't experience any temporary slows done for house keeping etc.
I like it, but the UI...
...is not perfect.
For me it is too bright, everything rounded just silly. (I found a dialog in preferences with two adjacent touching buttons with rounded corners...)
The transparent menu is bad idea. I have multiple desktops with different background, so in one desktop I have white text in menu, on another one black text, this just hurts the eye. I know it can be "disabled" in accesibility, but this should be a separate option just for the menu.
The toolbars are too large, the right aligned toolbar in finder is stupid. And the toolbars are not unified for apps. For example: finder/safari has new style toolbar, but preview/text editor has old style toolbar, strange.
First bug I found: cannot change weather location on the widget.