Not AI, but I studied it extensively for about 6 months. I was trying to port Emacs to JS, line by line, about eight years ago.

I love Emacs' design. I think the cruft is minimal, and pretty much every line of code I studied had a good reason for being there.

And I also think there's a lot to learn from studying how Emacs is implemented. Few people will probably do this, but it was a nice experience for me. I learned a lot about gaps, text properties, how buffers are implemented, how the eval function works (it's surprisingly complicated between buffer-local variables and thread-local variables, but it's hard to think of a simpler alternative), and how intervals are implemented.