We used ours this afternoon! :) My husband was fishing with a friend and our 3 1/2 year old, so I went hiking with our 21 month old. We were on trails but the trail goes up with a drop-off to the creek. The "backpack" we have is from Target--I can't remember the brand but it is a beige puppy that clips in two places across the chest. The "leash" is removable. I left it looped in my belt loop most of our hike and just clipped it on when we hit the high ledge area so he could be free without plummetting 40 feet into moving water (which would really stink).
I like that the "leash" part is removable because I can stow a cup of water in the backpack (great to keep your kids hydrated when you're out!), and if I am holding the end of it, I am usually also holding his hand or letting him trot ahead without any danger. We have also used it in crowded areas where kids can get away from you quickly.
My father-in-law reminds us (from 2 1/2 hours away) to use it anytime we're going anywhere with crowds, so I'm glad they think it is a great idea. They know what sociable, uninhibited boys we have. We don't use it on our older boy anymore since he is pretty good now, but we used it last summer hiking in Acadia as well, since again I wanted him to be able to clamber around freely but did not want to have the coast guard fish him out of the Atlantic. Anyway, I LOVE having one, we let the boys use it as a backpack when we're out which cuts down on what we have to carry for short trips, and I know they can't get themselves lost or fall somewhere. The harness is comfortable, adjustable, and safe for them to wear. I also find people are more likely to see me out with it and remark one what a great idea it is--very different from the wrist lines, which I would never use and hate to see. Before you use it with the "leash" (I use that for lack of a better word), use the backpack part a few times so he can get used to it.