I donīt use those.I have 2 regular 4ft leashes and when we get down to the beach I let mine go without clipping the lead off (so it trails along behind them).
If I see another dog, I shout "Come" to Milo and Bella, and they run back to me and I grab their leash.It takes all of two seconds, but I must admit that itīs because theyīre very obedient when out on walks so we donīt have a problem with this method.
If they were runners or they didnt come when called, Iīd deffinately use a flexi leash so they they could experience some controlled "freedom" on walks