One other thing not mentioned is the possibility of injury due to the cord on the flexi. I've seen injuries to both people and dogs, rope burns ranging from minor to pretty serious. I "heard" of a finger being severed, but that could be one of those rumor type things, I have no personal knowledge.
I would urge any of you who do use a flexi to get one that has a flat ribbon type cord rather than the small diameter nylon cord, much less likely to get a rope burn when either tangled or trying to retrieve your uncooperative dog.
I understand the appeal of the flexi, and there are places where they could be appropriate, but a crowded campground or an area where potential dangers to your dog are present probably isn't the place. Most dogs are not trained for a recall when they are presented with unusual stimuli.
Kaylee isn't trained for any recall :), thus is never off leash (6' when in a crowded area). We're working on it, but it's a process.
Doug, DVM