I bought one as I drive around 400-500 miles a week to and from work and even though I've not needed the footage yet (thank goodness), I still believe that it was one if the best investments I've made. It really would have come in handy when I had my accident as most if the time you can never tell what exactly happened. I wouldn't want to run the risk of not having one if you're a regular commuter personally, and I would make sure that you get one with half decent night vision if you drive in the dark too.
Have a look online at the specs for cameras. I use a Kenwood one which is great, but it's in the higher price range, but you definately get what you pay for. Also like others say you can get it wired into your fuse box. If you're not happy doing it yourself then I'm sure there will be garages or mod workshops in your area that do it for you.