I have 2 of the Flexi - Comfort 1 size and love them.
When I'm getting the puppies ready for their walk I hook them up and set the lock. I let them drag it around in the x-pen I have set up by my front door while I grab my keys and stuff. It freaked them out the first time or two (they kept trying to get away from it) but now they're used to it and I don't worry that they will freak outside should I happen to drop it
They have also learned to listen for the click of the lock unlocking because they know that means we're coming to a grassy area where they will be allowed to run. Believe me, we have gotten tangled many times but I've gotten the hang of it by pretending I'm flying one of those tandem kites - LOL!