I have a Road Angel 'Vantage' in mine. The model has just been replaced by the 'Gem', more details
here. The GEM has just won the Auto Express award for 2012. They cover fixed, mobile, average speed, schools/20mph zones, black spots etc, and you can add your own so if you find they have a 'favourite spot' on you route to work then you get a reminder to wake you up - and you set the distance from that point.
The units do work well
providing you don't have a Climate Comfort Windscreen on your car. These block the GPS signal so you also need a separately available GPS aerial (which they also provide) which you simply attach with a bit of double sided sticky tape on one or other side of the rear view mirror mount, close to the top. BMW helpfully added a pair of 'zones' there for the purpose...
With regards to the laser guns, they have an 'approved' range of up to 1,000m and a usual 'operating' range of 100-300m, so as pvr says, by the time you've seen it you've had it in a lot of cases. It depends on where the operator sets the beam for the van mounted ones. The usual 'aiming point' is a headlight or front number plate. The reading takes 1/10sec on average.
There are 'countermeasures' BUT, every gun in use by the UK Police Forces now has a detection system and will instantly tell the operator if it is being 'countered'. That in most cases gets you stopped and the offence is 'Obstructing an Officer in the course of his/her Duties' or 'Attempting to Pervert the Course of Justice' - my brother is a retired Chief Super (can you tell..

). If convicted you could lose your car and liberty, and will at least have to remove the 'garage door opener'. If you 'counter' a mobile Scamera they take your piccie anyway and then might pay you a wee visit! Given the cost of said devices it's not worth it. As usual the offence is not buying, owning or installing them, it is
using them. As pvr said, there is no specific motoring offence, hence using these two listed above. Norfolk are very hot on them!
I've used Road Angel hardware for many years (after getting caught) and found that unless you put the laser pickup by the number plate etc, they rarely trigger. More and more devices no longer bother with that part now.
These things are not cheap, and if you have one with a 'data over mobile network update' capability then there is an annual fee of about £60. These devices have saved me more than once, not for doing silly speeds, but when
I've been in 'auto-drive' mode..
The old radar guns were easily 'countered' and in a 'former existence' I was involved in a few successful prosecutions under the Wireless Telegraphy Act, where the offence was 'deliberate interference' and use of 'unlicensed equipment'. Both got you a fine and slap on the wrist. Oh and we took the toys away too...
