The nice thing about org-mode is it's as involved as you would like it to be. Sure, the beginning is a bit steep, but if you're already using Emacs, it will be a breeze, and you can keep it light (as pnathan is doing). I've been slowly inching my way into using it over a long period of time; I have to admit, it's quite involved for me, but it never felt forced, it was always "cool, that works nice; but I wonder if I can X?" and with org-mode, you can almost always do X. Or not, if you don't want to. Unlike a lot of other "productivity tools" it's very flexible.