Look, some people love them, others hate them. I think most people fall into one of the two camps. Child leashes seem to polarise people. I think its because the people that hate them can't understand why you would want to treat your child like a dog, but the people that love them realise how valuable they can be at keeping ultra-adventurous kids from running off.
Personally I don't much like the look of a child on a leash, however, when we were first thinking of taking Bendy on her first overseas trip, we wondered whether it would be a good idea to have one. The Bookworm is a very compliant child, and if you tell her not to do something, she will comply. Bendy, however, likes to see what she can get away with. For her, we wondered whether a leash might be really useful in busy places like airports and on New York sidewalks.
At the time, I only knew of the sort of leash that is just a harness. I don't like the look of them, and I can't imagine a kid does either. So I thought about how I could make a leash a little more inviting and even desirable to a child. I decided to make a harness with a built in tulle skirt with sparkly sequins sewn onto it. For a little girl I hoped that this might make it more desirable. It did. We tried it out at the shops with Bendy before we went on our trip. She did like it (a bit). However, by the time our trip came, we realised that she really didn't need it, so I don't think we even ended up taking it with us.
My nephew though, has a backpack type leash that he loves. The backpack is in the shape of a dog, and he calls it Scrufty (I think this is the name of Bob the Builder's dog) and he loved it. He no longer needs it as he has matured. But it was really useful when he was younger.
Its in the same style as the one pictured at the top of the post. I had a look on Amazon, and there's actually a large range of different animals that you can have: a lion, a horse, a panda, a pink poodle, a cow, a giraffe, a monkey, a koala, a lamb, a frog, and even a unicorn. And then there's a range of Disney characters including Piglet, Eeyore, Winnie the Pooh, Tigger, Pluto (who looks like his neck is broken - nice touch!), Mickey Mouse and Minnie Mouse. Pretty much something for all tastes... So they are quite a bit more palatable (for the adults) and desirable (for the kids) than the plain harness type (which are also still available) I used to see when I was younger.
For travelling with kids, they can give peace of mind. Though you'll have to ignore the dirty looks that people who find them offensive might give you! For me, I don't think I'd care what other people thought if I felt it was going to keep my child safer than without.