We have a harness for DD1 and she loves it. It is actually just a backpack that has a short lead attatched. DD1 loves that she carries a few of her own things when we are out. DD1 is a very confident little girl and has no worries if she can not see me. When I go out, it is 99% of the time without DH as he works alot. I can't just pick DD1 up, put her in a trolley or pram because I also have DD2. The sense of calm I have that she has to stay with me, can not run away to any danger, no one can take off with her.
It actually only took DD1 a few times at wearing the harness to realise she could not walk off and now she is great at staying near me. Personally I think they are great and god help anyone if they tell me that I treat my child like a dog. I bet anyone who has ever had their dog run over while out walking wishes they had harnessed their dog.