My wife has had one for years, which we pay something like $15/month for a really small number of minutes. It's mainly an emergency phone for her ... she has a 45-minute commute to work, down a freeway that pretty much is the only way to get from here to there (and thus, if there's an accident, you're royally pwned), and needs to be able to make calls in situations like that. It works for her.
As for me ... I don't see an absolute need for one. Most of the time, I'm at home (by a landline) or at work (by a landline). I don't want to be one of these people who talks on the phone while driving, due to safety concerns. So, the only other time I'd really need it is if I'm out-and-about and need to get to a phone (like calling the wife to figure out if I should really buy that big shiny or not).
Thing is, it's almost becoming expected behavior that one has it. When I was out in Portland, a couple of weeks ago, I was trying to meet up with a colleague in an unfamiliar place. He had a cell phone, I didn't; if we both did, it'd be a lot easier to just call each other and say "I'm under the arch, where are you?". A month ago, I had to fill out an online web form, and got stumped when it required that I submit a cellphone number and I didn't have one to give.
In my postal mail yesterday, I got a deal from AAA for an extremely low-cost cell. Free (cheap) phone, $10/month for network access with no minutes, $0.25/minute, and no long-term contract. If it turns out I start actually using it, I can upgrade to 100 min/month for $20/month total, with much bigger chunks of minutes available as well. It appears to be a completely reputable company.
So ... is being part of the 21st century worth $150/year? There are a small number of situations where it'd be incredibly convenient ... but I'm not sure, even at $150/year, that it's money well spent.