I do have a back pack with a lead but haven't used it yet...my mum bought it for me but I have been in two minds about using it.
For me, DS was a runner so our backpack leash has been wonderful. DS loved it as it was his "monkey" and only used when he was walking so it was a special treat for him. We've had a few comments about keeping kids on leashes but better to keep him leashed than have him run over or abducted or lost. We haven't needed to use it for a long time now as he has learnt to stick close and will hold our hands or the pram/trolley. He gets one chance even now at free walking. If he tries to run off, he must hold our hands. If we are somewhere near cars, he must hold our hands at all times, no free walking. For DD, we haven't needed a leash, yet. She sticks close without it. She is starting to test the boundaries so it may have to make an appearance soon.