I started carrying one when I started travelling frequently for work. I can't tell you how many times I've used it for directions, to call and say I was going to be late to a site, to call from system rooms that didn't have phones, to call Rusty so I didn't have to pay the hotel an arm and a leg for a 10 minute phone call... One of the biggest things that I like about the cell now is that if I break down in the middle of nowhere or have an accident or an emergency, it's available to me.
Also, I thank God Rusty had his cell on 9/11. If it weren't for that, I wouldn't have known if he was ok or not for hours.