I joined, but then deleted my account. At first you do get a sense of community. If there was a siren in the middle of the night... someone will be reporting as to what it was. At times there are even more immediate alerts as to possibly questionable people roaming around the neighborhood. You get current reports of theft, car break-ins and almost anything like that. In that sense it can add to a feeling of security. People share their experiences with the various trades. If you need a good electrician, plumber etc, you can get a feeling for what is out there and who's doing a good job. There are MANY positive reason to have an account.
But, there are some who feel like they're the pillar of the community and are quick to respond when they don't know all the facts. Recently, I was told by my neighbor who does have an account that there was an issue about someone driving in the neighborhood in a dangerous manner and a mob mentality seemed to be taking over (at least online). The gentleman who all this was directed at happened to be a member of the website. He politely introduced who he was and then proceeded to point out how ridiculously wrong the original story was. People finally realized that they had overreacted and apologized. Something along these lines happened to me. After a brief and cordial exchange with a resident of our community (at my front door) who apparently didn't like what I had to say, I was being apologized for by others on the website stating things like... "That is not representative of the people of our community", "we apologize for that" etc. After reading how everyone was willing to take one's side without hearing both sides turned me off to the site and I immediately deleted my account.