I’ve used Pine as my email client for, well, pretty much forever. I use it because it’s fast, powerful, stable, and very keyboardable. (I hate the mouse.)
However, since I work at Google, I’m constantly bombarded with people who ask me why I don’t use Gmail. After hearing the nth person brag about how much it increased their productivity, I finally broke down and tried it. I didn’t expect much, since I’ve never liked web-based email clients. However, I made myself use it as my only email client, for a month, to give it a fair shot.