I like Stage Manager, it brings back the single window mode from early versions of Mac OS X and removes the need for a defaults hack to get it working.
I have not found it especially useful on my MBP, but it is a huge improvement for me on the iPad with keyboard and mouse. It feels much more natural now to have two windows open and quickly switch between apps. I used my iPad for about 80% of my work now, up from 10-15%.