Rather surprised at the range of comments to this topic, some thinking it is downright dangerous, in other than open road/motorway cruising and others thinking, it's the best thing since sliced bread.
I'm in the sliced bread category and use it quite a lot.
As others have said, it's very useful in avoiding over-speeding, particularly in 30 mph areas.
Generally, i find that the actual speed as shown by the satnav, is perhaps 1- 2 mph down compared with the set/indicated speed, which of course is an additional safety margin.
We have near me a stretch of 30 mph road, monitored by average speed cameras and find it somewhat annoying to be behind a car doing perhaps 24-25 mph, which to my mind is bit overzealous.