OnStar, now available on non-G.M. cars in a rearview mirror package, is a seamless brew of cellular, Bluetooth, GPS, microphone, speakers and human operators that offers great safety features. Most people probably do not think of General Motors as the living definition of a technologically nimble company. But deep in the G.M. archipelago of cars, parts and wholly owned subsidiaries, there sits a technological gem: OnStar. OnStar is a seamless brew of cellular, Bluetooth, GPS, microphone, speakers and human operators. Its feature list includes elements aimed at both safety and convenience. For example, if you crash, sensors wirelessly alert a 24-hour call center staffed by 2,000 OnStar operators. they immediately talk to you over the built-in speaker. if you do not respond, or if you say something like “My legs are broken in six places,” they automatically send an ambulance. they know exactly where to send it; they can see where you are; and they know what kind of car you are driving. According to OnStar, 4.5 million G.M. owners so far have liked their free six-month OnStar trial so much that they signed up to pay for it. For 15 years, you could get OnStar only in a G.M. car. Now, however, OnStar is available as a rearview mirror that goes into almost any car. It is called the OnStar FMV, which, we are told, stands for “for my vehicle.” The OnStar FMV, which is available only in the United States, may be the world’s most useful rearview mirror, but it is probably also the most expensive: $300 plus installation. For my testing, OnStar loaned me a Toyota Camry with the mirror preinstalled. (That is right. G.M. gave me a Toyota.) After the installation, an unobtrusive wire runs up the windshield from the mirror, around the door molding and down to a control box. The mirror itself looks great, at least from the front; only a few nicely integrated buttons distinguish it from the original mirror. from the back or the side, you can see that it is chunked up with internal components. Drivers in the United States also have to sign up for the service, which costs $200 or $300 a year, depending on the features you want (more on this in a moment). You can also pay monthly: $19 or $29. clearly, OnStar imposes what you might call a superdeluxe, premium fee. Fortunately, for that, you get a superdeluxe, premium product. Every plan includes the famous Automatic Crash Response — that business about OnStar reps’ calling you when you are in an accident. The sole difference is that the “embedded” OnStar system (built right into G.M. cars) also knows if your airbag went off, if your car overturned, how fast you were going and which way and whether or not the car got hit more than once. FMV, on the other hand, knows about your crash solely from the motion sensor in the mirror. That does not seem as good, but what do I know? What I could test was the human operator function. as you are driving, you can press the OnStar button for an immediate connection with a live operator. These operators are well-trained, personable and sensitive to the fact that you are probably in a car and in a hurry. You can ask them all kinds of things. You can ask for the nearest gasoline or cash machine, a weather report, an address or a phone number. You can get roadside assistance when you run out of gas, get a flat tire or need a tow. But best of all, you can ask for directions. it is amazing. You press the OnStar button. You hear a couple of rings on the speaker, and then your call is answered in 15 or 30 seconds. “Hi, this is Phoebe. Where can OnStar take you today?” “Oh, hi. I need directions to LaGuardia Airport.” “All right, sir, I’m looking that up for you. Got it. O.K., I’m sending those instructions to your vehicle.” “O.K., great! Thanks.” “Thank you, sir. have a great day, O.K.? and drive safe!” Eight seconds later, your mirror starts talking to you. “Directions are now being sent to your vehicle. Your destination is 47 miles away. Your approximate travel time is 1 hour, 7 minutes. Start out going north on Chicken Hill Road.” And now it is just like having a Garmin GPS unit — same voice, same quality of directions — except there is no screen and no map. That means you do not get the ability to spot alternate routes visually, and you do not really know where you are. On the other hand, audio-only means you can keep your eyes on the road, you can “program” the GPS while you are driving, and you never have to specify your starting location or direction. it is a soothing, satisfying thing having a human lifeline one button-press away. it is easy to see why people get hooked. The mirror also acts as a voice-controlled speakerphone. it either pairs with your existing cellphone over Bluetooth, or you can pay (heavily) for cellular service — for example, $40 a month for 100 minutes. That is right: you can now be the first on your block to have a rearview mirror with a cellphone number. The $200 a year plan gets you emergency services only. The $300 plan adds unlimited driving-direction, information and lonely heart calls. OnStar notes that during natural disasters like floods and hurricanes, it opens all of its services to all OnStar customers. OnStar, in short, is just incredibly well done. it is efficient and reliable. it is not nearly as overwhelming as car computer systems like Ford Sync or BMW iDrive. Its operators are not the usual sullen, get-off-my-lawn drones of the customer-service ilk. they are bright, interested and people-oriented. Of course, there are plenty of reasons not to get OnStar FMV. it is expensive. You could duplicate some of its functions with a suction-cup GPS unit and a Bluetooth dashboard cellphone system. and at this point, there are a few car models it still does not work with. Note, too, that the FMV’s plans cost the same as the embedded OnStar plans, but you do not get as many features. The embedded OnStar for G.M. cars also gives you Remote Door Unlock (the agents can remotely unlock your car), Vehicle Diagnostics (reports on the car’s major operating systems), Stolen Vehicle Slowdown, Remote Ignition Block and an iPhone/Android navigation app. But one thing is for sure: if you can afford OnStar FMV, you will love it.