I think they're a good idea
A couple of my cousins have them and they're handy for kids who walk home from school alone or are out playing in the neighborhood with friends.
I only have one concern w/ kids & cell phones. A little 8 yo girl lives in my apt building and she walks home from her dance classes alone in the dark @ 7:30 everyday. She is ALWAYS blabbing away on her cell phone to her friends with her ear piece in. I think that if a parent is going to give one to their kids, they shouldn't program their friend's numbers in there because it is unsafe for kids to be walking home alone in the dark in the 1st place, plus with that added distraction of chatting away, it becomes much more dangerous.