I now use one on my commute.
Last year I was involved in an accident when a car pulled out in front of me while I was doing 30mph. I have no recollection of the accident, an hour preceeding it, or the 6 hours after. I was off work for a year and I would have been out of pocket if the driver had just driven off, so I film the commute and any rides where I am in heavy traffic (in my experience that is where most incidents seem to happen) I rarely bother to look at the footage and just overwrite the card when I get home.
Edit to say that the accident happened on the quiet country roads of my commute rather than the city centre, so it goes to show you do not know where or when something will happen, but I am happy to have the camera just in case.