I had one when I drove a taxi, now I don't--but I'm thinking of activating a new account, due to it being at least seemingly cheaper than conventional long-distance rates (and I LOVE to call my sister and gab for hours with her--we're very close). I have to weigh the rates closely though, due to one being a flat rate and the other a rate based more on time.
For taxi driving, they are quite useful for giving to regular customers, who can then call you when they need a ride, rather than call a dispatcher, get questionable service, and not necessarily give YOU their business (but whoever is dispatched). They are also good for locating a customer who has an ambiguous address or something (provided they, too, have one). The cell phone sure helped my business (taxi driving is an independently-contracted business of each driver). Obviously though, if you cannot drive well while using these devices you shouldn't. I could reasonably well, but I tried to keep on-the-road conversations pretty short and to-the-point just the same. Today, I would buy a headset, too.
I can see where they would be convenient for road emergencies, i.e. staying in your car and dialing AAA or whatever rather than walking alongside the freeway to the next exit where there is (hopefully) a gas station with a pay phone.
The thing I hated about them is that they are an electronic leash--if people know you have one, they tend to think that that entitles them to unlimited access to you. And I tend to be the type who doesn't even answer my home phone all the time--sometimes I'm in the middle of something, or otherwise don't feel like talking, so I'll wait and check for a message in case it's an emergency. I suppose I could do that with a cell, too, it's just that, like I said, people expect unlimited access (versus the excuses you can give for not answering a regular phone, I was out, on the john, or whatever...).
