This emacs-wiki-discuss posting by Paul Lussier has a great description of how planner.el sucks people in “first taste is free”-style:
As I started in using planner and emacs-wiki, I very quickly became addicted. This was almost literally, an overnight conversion of my life. I no sooner started using emacs-wiki/planner, than I found myself using erc. Reading johnw's README for planner led me to his site, where I discovered ledger (John's unbelievably powerful financial app.) and eshell. Then came (in no specific order) w3m, muse, remember, bbdb, and last, but not least, gnus. The last three are significant. I had been mostly happy with my prior e-mail environment of an mh-backend based e-mail solution for the better part of a decade. But there was no way I could hook that in to planner, and after a several months of resistance, I attempted the switch to mh-e which, as you may remember, didn't go so well :) So, now I'm on gnus, and almost every facet of my life is now hooked into emacs. I've learned more about emacs and lisp in the past year than I have in the past 10 years.