I have a 6-month old Staffordshire bull terrier puppy and a 5-year old lab mix. I wanted to see if the leash coupler would cut down on the puppy pulling. Since the pup weights 21 pounds and his sister 64, I was worried that she would drag him around. In fact, the coupler balanced them quite well. The bungee section also minimized strain on my back. The coupler is well made and has a quality feel to it.
The only downsides are due to my particular application. When dogs are coupled, you lose the ability to correct dogs individually as you would through two leashes. Thus when my puppy wanted to get at some palm nuts (with indigestible pits), the only way I could hold him back was to grab his section of the "Y." Likewise when both dogs were getting in to something nasty, I could not pull them away as I would with two leashes. I think that when the puppy is an adult the coupler will see use again.