Neighbourhood Speed Watch, good or not?

For those of you who don't know what the above is, neighbourhood speed watch are people who stand on a street or road and clock people's speed. This usually happens in a 30mph speed limit zone (sorry I'm unaware of what that is in any other country). They have either stopwatches or the speedo's that the police have. They take images of the people going too fast, in some cases 1mph over the limit. The police then give you, I believe it's 3 cautions, then take action. So do you think it's good or bad that they do this? I think it's good that community members are ensuring that their roads are safe.