I got one from ebay a couple of years ago - china color cam special at a good price.
It came with a 2GB mini card and just rolls-over as it fills - so I always have the most recent 2 hours of video.
I wired it into the 12v so that when the ignition is on, the camera is on. (That is MUCH better than the 'plug into the cig lighter' that they all come with as standard.
I have had the usual instances of dummys pulling in front of me and then hitting the brakes, but so far no accidents.
I do hope I NEVER need it, but just like the insurance coverage, it is 'just-in-case' there should be any questions.