There was another thread like this not to long ago and I remember somebody with something that went sort of like this:
You hold your hand up in the air for an hour and tell me you're not cranky!

that is the funniest thing ever!
I used one for ds when he was little (used to bolt in parking lots), but the girls never needed one. Most of them are so cute at WDW, and if people have a problem, they can get over it.
The only one I ever saw that I was a bit appaled at was this Christmas at WDW. Mom had 2 kids with her - not toddler age more like 7 or 9 and typically developing by the look of it - they had clips attatched from them to her - not the kiddie leashes. They were that plastic twisty springy stuff like you can use for your keys? (I have one on my key chain that is a loop and you can put it on your wrist, some I've seen are a longer plastic part with a clip). Anyway, they were like 5 feet long! and the kids were running around, crossing behind her, in front of her, running in front of other people (which in the Christmas crowds is pretty amazing). It seemed like they were just clipped to Mom so Mom didn't have to bother with them. I felt like the currator in Night at the Museum.."CONTROL YOUR YOUNG!". That was more of an instance of odd parenting I think.
I think it's totally appropriate for your basic under 5 set and maybe a little older - depending on your child.