I just got the Monkey backpack harness for my son as well (Mostly just to say hey Honey, there's a monkey on your back!) It's funny that people have a problem with this. Most of the time they get it when I explain it's actually SAFER for the kids to wear these then to dart out in traffic and it's actually quite cruel to make your kid hold his/her arm up to your hand for long periods of time (you see how long you can hold your arm up, it fucking hurts)
depends, with my 2 year old, we got it to where he stays close, holds our hands from the car to the door, will either climb into the double stroller on his own, or will hold onto the side of the cart (or sit in the cart), and I guess if he gets cranky or is really tired I carry him (which isn't too bad since he's like 23lb). But the point is by making him actively participate in his containment of sort, rather than sticking him on a leash you kind of get the kid trained down to where a situation like that is less likely to happen.
Outside though where there's traffic, he's either holding hands, in the double stroller, or being carried. I swear if I stuck a harness on him, his only desire would be to get out of it, or frantically pull in every direction possible, and then possibly lead to a full out melt down, least until I get the chloroform ready.