Most of the comments in this thread are from people who have apparently never used one of these devices, nor read the websites for the companies that offer them.
We are a 2 driver 4 car household. I was looking for a way to reduce our insurance rates, since we don't drive any more than when we had 2 cars, but the premiums went up proportionally by car. So I tried the Progressive snapshot devices in 3 of our 4 cars (wife refused, but she drives a ton and hard stops all the time anyways). 2 of those 3 are what I would consider occasional use. I am able to adjust my driving habits around which cars have the device installed.
-The Snapshot devices do not have GPS. They communicate by cell tower, so they could track your location that way, but I don't think they do.
-The devices only track how much you drive, when you drive, and "hard brake" events
-A hard brake is defined as slowing more than 7mph per second. This is actually pretty soft, and you will trip it even while driving cautiously. Particularly if you catch a yellow light, or drive at all in bumper-bumper traffic.
-The "when you drive" part, they penalize for late night / early morning driving.
Based on some algorithm, it then gives you a percentage discount on a per-car basis.
Results:
-I'm getting 8-10% discount for cars that drive on average very few miles (~10) per week, with <2 hard stops among those miles. This seems like a bad deal to me, I'm paying a lot of money to insure cars that sit in the driveway.
-The devices don't always work, even when plugged in sometimes will not power up. When this occurs, it isn't tracking your driving. It may decide to turn on part way through your trip.
-Having the device in the car is a safety issue, if the thought enters your mind to run that yellow/red light rather than having it beep.
-They advertise as though you get a 1 month trial, which sets initial discount, then a 5 month monitor period, for a total of six months. What really happens is they will make you keep the device hooked up until your next renewal, so it could be 11 months if timed poorly.
-I suspect they have an 8% floor on the discount offered for using the program, based on watching mine fluctuate up and down. This may roll into an 8% premium increase at next renewal, will see.
In the end I'm not satisfied with the discount provided and will be shopping elsewhere.