I have one in my work vehicle, records to a micro SD card on a continuous loop of about 72-hours. The recorded data includes exact location coordinates and vehicle speed by GPS, so it could be used as evidence (for and against) in the event of an accident - however being totally honest, my driving style in that vehicle reflects my awareness it's capturing everything I do, so I probably 'behave' better than if it weren't there....
You can also override the automatic recording loop to capture a particular event that you want to keep footage of, and it also provides a 'kml' output file to Google Earth that allows you to plot your travel movements, can be really useful if you need a reminder exactly where you saw that interesting car stashed behind an old barn!
