Car Cams - Are they worth it?

I am thinking of getting a Car Cam after getting the blame for a minor car incident which was not my fault - the other party had a friend lie for her (which I can't prove) while I only have the truth.

Are they worth it? From reading this forum I found that some leave theirs in their cars all the time, the security of this would worry me so are they easy to put up and take down?