It's quite a scene. I did it many years ago and have no desire to do it again, but as @
leob1 says, it's a good thing to do once. It's very crowded, especially in the first few miles. I remember some very wobbly people who rarely ride bikes and had no business being on a mass ride. Then there were the racer wannabes who also shouldn't be there. One of them yelled at the others, calling them "amateurs" as if it's some kind of professional event.
Being in a big group ride can be exhilarating, but there are other group rides that are less mad. The best feature of the 5BBT is that you get to go on roads that are normally only for motor vehicles. The most memorable for me are the Brooklyn Queens Expressway (BQE) and the Verrazano Bridge.
Transportation Alternatives organizes several rides throughout the year. I like them better. They're much cheaper. The Tour De Bronx might be the most fun. There's a lot of camaraderie among strangers. My wife and I went this past October and had a blast. Several years ago, two years in a row, I did their NYC Century ride, where we do 100 miles through four boroughs. That ride has shorter routes, and I think they are 15, 25, 50, and 75 miles. 100 miles is kind of crazy, and I did it for the challenge. But I think it's the only one that goes through the Bronx. There is something special about riding with thousands of people through the Bronx that the other boroughs don't have. We were greeted more warmly there by residents than anywhere else. The Tour De Bronx is free! Sponsors pay the whole bill!