After getting a ticket on the A1, where the speed limit dropped from 50 to 40 and I didn't realize, I have for the last 15 years used almost exclusively a Road Angel. I currently have their ''Pure' model which has an ultra clear display visible in even bright sunshine, and as well as showing the correct speed limit, it also shows your actual speed (albeit with a second or so delay) which is useful when cruising on a motorway, in fact you begin to use that display as a back up speedometer. Whilst the unit can give all sorts of audible warnings including spoken word, these can be disabled to different degrees in the set up menu. It warns of all camera types including traffic light cameras and bus lanes, and updates continually over the air. Meaning that if another user or yourself spots a camera can you can send or receive alerts of 'live' camera van locations. Last year they introduced variable motorway speed updates, which costs a little extra annually in terms of subscription, but this is of 'variable' benefit as the displayed limit doesn't always correspond with the speed limit on the gantry, the device also has Lazer detection, but as 'jasst' pointed out, if it goes off it will usually mean it's too late!
The unit has a built in rechargeable battery which will give you 3 1/2 to 4 hours running, and as I don't like wires running across the dash, or mounts stuck on the windscreen or dash, I have it mounted to a compact auxillary phone battery power pack which the unit can plugin too doubling the running time, and it stays put on the dash top courtesy of a non slip mat. All this means when the car is parked everything fits in the door bin or central cubby box, and it can be up and running in ten seconds.
The subscription £100 pa or about £250 if you go for a three year subscription, and the device is about £300.
I understand they recently introduced a plus model which I think has dash cam functionality, although I'm guessing it will need to be permanently wired.
All I would say, I think it's absolutely vital, as there have been plenty of times when in an unfamiliar area it has saved my license, and I have not had a ticket since.