First of all, I absolute love it. Usually, I am "meh" when it comes to UI changes and it takes me a while to adjust, but I felt immediately at home with Big Sur. It's very slick, undoubtedly Macintosh in spirit, while being more relaxed, pleasantly colorful and decidedly playful. And it's very consistent and expressive.
I also really like how much more useable space one is getting in apple like Finder and Mail. The UI is reduced to the minimum without removing any functionality, and it just feels intuitively right.
Neutral: I feel like some adjustments to contrasts etc. are still needed. Also, some Icons are probably not the final version.
Bad: some apps look a bit weird, for instance Activity Monitor. It seems they just built it with the new UI but didn't really tweak it. The toolbar is kind of forced and the table view looks rather of place.