So, I've been enjoying learning and using vi for the last couple of years. The beauty of vi, for me, is that its UI is a language of movement and action with a very uniform, simple grammar, and which is terse enough that the requisite memorization pays ample dividends in how much more I enjoy working with text (by avoiding boring repetition and eliminating micro-hassles, like that half-second annoying wait while you scroll down the screen).

(Note--I don't claim to have expert knowledge of vi, but I get around decently well: comfortable with limited '@' macros and regexp search-and-replace within files; frequently use multiple buffers, tabs, and windows; get around pretty well in the file browser; understand the grammar of actions + movement + subject (as described so aptly in this beautiful SO answer); and had some pretty sweet debugger and ctags integration going with PHP.)

I wonder if some emacs folks could take a swing at explaining what emacs does brilliantly, or sum its strengths up in a phrase or two. Spare me the talk about productivity; I'm more interested in conceptual clarity. Lisp-centric answers are okay; I'm learning Scheme on the weekends, and would pick up emacs for that alone (have been using Racket).