My wife purchased some online. We had a long discussion about it. Right now, our twin 22 month-olds are VERY active and LOVE to wander.
Yet they don't really listen very well. (In fact Cole, tends to do the exact OPPOSITE of what we ask, even if we raise our voices - he usually just laughs and thinks we're playing a game.)
Here is our reason (and probably the only time we'll use them...)
Our *almost* 4-year old daughter is starting Pre-K and needs to be dropped off and picked up at school twice a week. The school doesn't have parking that is close to the door (1/4 to a 1/2 block away - which is too far for her to walk alone we feel.)
Since I'm at work when she needs to be picked-up from school, my wife will need to bring the twins with. The school is on a semi-busy street and we feel like it's just too big of a risk to try to keep both boys walking with my wife each day corralled and not darting out into the street.
The school has a small amount of stairs, which would be very tough (and probably unsafe), especially in the Winter with snow, to pull a double-stroller up and down.
The leashes haven't come in yet...they are actually pretty cute, they look like teddy-bear backpacks. I'll report back once we use them a time or two and let you know how we think they work.
I've prepared my wife for the "looks" and comments she might receive using these...much like this woman did in the ABC News show "What Would You Do?"