They are only as good as the as the weather/windscreen and camera allow. Having said that 3 years ago leaving london on the A4 i was driving a 44 ton truck for security reasons due to the nature of goods i was carring i had a camera linked to my laptop and looped 20 minutes record time,4 cameras front,back,left and right side.
Just as i was approaching a junction where the 3 lanes would only just get 3 cars side by side or a truck and a car i had an accident. I was in the middle lane and half of the inside lane due to the size of my vehicle the outside land was for turning right only,I was overtaken by a black BMW which then swerved in to the lane i was in quickly followed by a Volvo estate car. the traffic in front slowed and stopped and as a conscuence i hit the Volvo estate car. Needless to say at 20 miles per hour weighing 44 tons there wasnt a lot left of the said Volvo.
To my surprise when the dust settled 2 police officers where stood waiting for me to get out of the cab, when i did get out i was arrestted for dangerous driving and damaging a POLICE VOLVO !!!!!!!!!!
Too cut a long story short,rear end accidents are the fault of the drivers who run in to you, and the police officers duly said i was at fault,being a undercover camera car they claimed tohave footage of my driving being bad.
the Police car was on duty follow a suspect vehicle and i happened to take them out as they put it.
open and shut case. Until i happened to say that i have the last 15 minutes of my driving on film although not linked to a speed readout the police chief who came out to the secne of the accident asked if he could view what i had recorded and promptly released me from custody of the other officers after viewing the recording.
A year later i was shock to learn that what i had recorded was used as evidence in a criminal case against the driver of the BMW along with several others that had commited driving offences caught on my cameras.
The company's insurance rep was quite pleased that i had proof the accident was not my fault and as far as i know London City Police paid for the damage to my truck after seeing the recording i had made
i still have the footage of the day and the pictures of the fubared Volvo Police car i shall dig them out and post on here.