I joined our neighbourhood watch.
The best thing is a list of emails of others involved in the scheme: then if there's anything "strange" going on in the neighbourhood, word rapidly gets around.
The worst thing is that the meetings tend to gravitate towards car-centric interests; lack of parking, resistance to RPZ, wouldn't it be a good idea to turn the local streets into one-way, etc.
It doesn't take up a lot of time, really, and on balance seems to be a Good Thing. And you get to know your neighbours, too.