Do you want an insurance company 'spy' in your car? Maybe so...

More and more insurance companies are experimenting with data-capture devices for their customers. In some markets, certain drivers can now not get insurance if they don't have such a device installed. But what are they, and should you use one?
There's a couple of different variations on how the insurance companies collect your driving data but the most common one is a dongle that plugs into the OBD-II port on your car (under the dash). The dongle has a 3G or 4G cellphone radio in it (for getting the time and reporting back to the mothership), a set of accelerometers, and an onboard computer capable of storing certain data that comes from the car itself via the OBD-II port.
For the most part, the data that is collected is number of miles driven, times of day, g-forces when cornering and accelerating, and g-forces when stopping. The insurance companies have a weighting system that generates a profile of your driving habits based on these factors. That profile is what is then used to determine if you are due for a discount or not. Right now, it's mostly used for discounts, but you can bet the time will come when this system will also be used to determine if you need to pay more than the basic premium.
There's also a subtle different between the various companies and the way the profile is created too - some want you to have the device in your car for a month or two, and they generate a one-time profile from there. Others want you do have it in the car all the time.
In the US at least, the companies offer "up to" 30% discount in most cases. If they don't detect harsh braking, not too many miles driven and you're not out in the wee dark hours of the morning, you can expect to see discounts. (My wife received about 12% discount on her policy).
The problem is that the metrics they use to determine if you're "safe" are somewhat flawed. By far the most flawed is the insurance company definition of "harsh" braking - they generally consider that to be a decelleration rate of 3.13m/s/s or more. Most people can't visualise that, so try this: if you're driving at 30mph (about 50kmh), you come to a stop in 4.5 seconds. For a modern car in city traffic, that's actually pretty gradual but most of these devices sample data once per second, meaning that if you're stopping at that threshold, it actually records 4 "harsh braking" events.
A much better metric would be 'did the ABS come on'? As most insurance companies won't insure you if you don't have ABS now, it seems like a safe bet that a data monitoring device should collect ABS data from the OBD-II port. The same could be true for cornering. Hard cornering isn't dangerous, but if the traction control comes on, that event could be stored for your profile.
Ultimately it's down to you as a driver. You won't get the full advertised discount, just like you can't ever get the advertised gas-mileage of a vehicle, but as long as you're not a total lunatic, you should see some savings.