I've had a roadhawk fitted for nearly 2 years now. Runs all the time and I, generally, forget about it... until needed as "evidence" on 2 occasions now it has been useful.
It proved I was stationary when the idiot reversed into the side of me.. even though he was not visible, the impact, both vibration and noise, were recorded and the visuals showed me not moving. result .. the insurers coughed up in full.... even though it took nearly a year...

Second time was when I was traveling behind other cars and a prat decided to change lanes without warning (or looking) and took out the car 3 in front of me. No-one injured but the road blocked by wreckage. When PC Plod turned up I was able to playback the incident on the laptop - fortunately I had it with me - suddenly all attitudes as to who was at fault changed .. ...

I was asked to forward a copy of the clip, which I did, but never heard anything ... I'm guessing the prat copped a plea.
Expensive .. but IMHO worth it.