I HATE Flexi-leads (retractable leashes) and frequently rant about them but I use one. And I actually own 3 of them <gasp>!
THERE, I said it.
BUT with that being said (aren't there always caveat's for everything??), there are only certain situations that I use them in - potty time.
The issue is that people don't know how to use them properly, which is why I hate them in public where most people 'misuse' them. Like all 'tools', if they're misused/abused, they are just as dangerous as anything else. But they do have their place if used correctly. Unfortunately, many people don't use them correctly, and that is when you start hearing all the horror stories - dogs walking into traffic (especially at corners) and dogs getting tangled up with other people, other dogs, trees, etc. As any dog trainer (even me!) will tell you, you usually do not have control of your dog if it's on a long leash that far away from you. I cannot tell you how many times I've seen a dog round the corner seemingly without an owner, only to see the owner 15 feet later, trying to reel the dog in - yank, click, yank, click.
When I use a flexi, my thumb is ALWAYS touching the brake button, and my attention is ALWAYS on my dog and the surrounding area. Plus, I only use them in my immediate yard and neighbor's yard. I do not use them for walks, being in a public places, or training time. They are strictly for potty (especially in the rain so I can stand under the overhang, haha).
Should they be used in all situations? No. Does everyone use them correctly? No. But to say they're all bad is not the truth either. It's COMMON SENSE people! I know such a novel idea...
... it's enough to make your head hurt.