I've used the DriveCam, and it is great.... It is always recording, but events aren't saved unless there's an impact, or an event where the driver hits the button to save / record it.... It has front and rear facing cameras, to show both the driver and the road ahead. It also shows some of what is behind the driver, in a standard car, but you wouldn't see to much in an Elio or a truck where there is no back window.
You can see how they work here:
They recently changed their name after being bought out, and their website is:
http://www.lytx.com/