It can be very brutal, but everybody doesn't completely lose it and try to eat the brains of the guy next to them. Usually when people get knocked down or worse, everyone who saw it happen tries to help the person out. Nobody really wants to get destroyed.
But you do have to watch out for those people who try to start stupid crap in a pit. These are the same kind of people who go to bars or parties and try to start fights, just because they are trying to prove something. But usually everyone is just there to have a good time, so just keep your guard up and you should be okay. And don't just jump in head first, check out the scene before hand (learned that one the hard way; my nose has been crooked ever since XD).