That is true, its not so easy to fiddle a video footage and leave no sign, so its a good thing for everyone in my opinion. And also good if they do record witness statements etc. Then if nothing else, they have a resource they can recall to capture the finer details they may have missed during a home interview or at the roadside.
At the end of the day it will help everyone, you as the honest Joe (ensures that you don't get played by a not so honest officer), the accused, in that they can have evidence on their side should an officer be accused of inappropriate behavior. And of course the Officer, they can be reviewed much more effectively, they can use this for appraisals and as evidence when looking to secure promotion.
All in all I think its brilliant, however - are they allowed to turn it off? I think the answer should be no! If they all get them, I think they should not be allowed to turn them off EXCEPT of course in highly private situations (when they need to make themselves "comfortable" at a 'pitstop'! but even then I think that this should be used. every moment of police activity - barr relieving themselves, should be accounted for on video footage. I think that way, it protects the honest police, if they do a 12 hour shift, and have 11:50 of recordings and can account for 10 minutes at the local convenience, then brilliant. However, if they only have 2 hours of recordings, they are not helping themselves if an accusation is made against them.