I think curfews are a good idea, and there is little reason for anyone to be out after midnight.
On school nights, unless it was a school activity, they were home with us. Weekends, I think 11PM is very reasonable depending on what they are doing. We insisted on always knowing where our daughters were. They had a lot of freedom as long as they were honest and we knew what their plans were.
Kids can argue all they want but they really don't have the life experience to know what can happen in the late night hours. That is our job to ensure they are safe and I think 11pm is very reasonable.