It's pretty incredible. Good documentation, easy to use, but still very powerful and extensible. Every component and prop can be overridden and modified when necessary, but the defaults make sense too.

I use it for every project now and it takes only a couple hours to get a functional MVP up and running.

MUI is really what sold me on the potential of the React ecosystem. It's so much nicer than writing HTML when you can actually compose an entire app out of widgets in a documented, vendor-supported way.