I made the mistake and clicked yes to update to 12.01. If you don't like big changes then don't update. It's a messy disaster.
You can no longer place some of the menu items on the left side view area. The podcasts screen is a disaster with Recent updates, Unplayed Episodes, & Saved Episodes being at the top of the screen. Who even uses those? Now I have to manually scroll down to actually see the podcasts I came on for. Of course Apple doesn't give any flexibility to delete these above columns either.
The bar that shows how far you are into a song or podcast is absolutely tiny. You have to squint or move your head within inches from your screen if you want to skip ahead. Other than that, at least it seems somewhat faster which is an improvement because it used to be by far the slowest program on my laptop.
These are just a few complaints I've found within minutes of installing. I'm sure I'll find many more nuances in a day or so.