I used to be totally against them. I would frown on the parents using them.
Since being a mother and having friends with toddlers, I see that they are really a good idea and a good aid.

I was in newcastle yesterday and saw a toddler with one, only his grandparents were on the other end, and it did make sence, they we baby sitting for the day, and we quite elderly, so wouldn't be able to run after him. It was more for his safetly than not being to control the child.