I have used one, when travelling with a 20 month old, but didn't actually find it massively useful. It sounds silly but surprisingly having a lead doesn't actually make your child follow you and be compliant
And it's not a good look yanking on a lead. So holding hands the old-fashioned way is still kind of neccessary.
It was semi-useful when going through airports etc as a back-up safety thing whilst trying to juggle multiple bags, documents etc.
Out and about we used a combination of baby-wearing (on the back- Ergo etc) and a very cheap $30 stroller (that didn't really matter if it got banged up a bit, and was light weight and easy to lift up gutters etc, or fold up and carry in one arm).