As a general rule, I am opposed to them based on principle. It's like your child is a dog... that said, I have a friend who's planning to use one when the new baby is born. Her 2-year-old has no concept of roads (I was warned of this, and was STILL shocked when I watched him one weekend). For her not to use one could be dangerous for that child. She also has a 4-y-o and an mild autism spectrum 6-y-o, so there will just be too much going on for this not to be almost necessary.
unless they never leave the house again, of course!
