[endgame]

There's a subtle benefit that I haven't seen any of the other comments describe: the UI is made up of text, and I don't mean that in some curmudgeonly "things were better back in the day" way. I mean that all of the commands you use for navigating and manipulating text, moving between frames and windows and so on all use one consistent interface. With magit, I can be editing a file, hit Ctrl-C, g, then do some git manipulations and switch back to my work with a much smaller mental context switch.

And because all of the modes share that emacs feel (except for the vi-emulation, I guess), that knowledge is transferable across every language you work on, instead of learning a new (possibly proprietary) IDE for each new language.

[shiro]

Completely agree. I often got frustrated that the "modern" featureful IDEs don't allow me to incremental/regexp search in drop-down menu items nor to use set-mark-command + kill-ring-save keystrokes within dialog messages.