They are lethal. Further to Wolfie's point, there are sadly plenty of documented cases where sighthounds have broken their necks if their owners have been stupid enough to use them. Most reputable sighthound charities warn new owners about how dangerous they are. Sighthounds are especially at risk because of their phenomenal speed from a standing start and their slim necks, thus concentrating any pressure.
For any dog, I'd gladly see them banned tomorrow. No reputable dog training class allows them.
If you are walking on a street, your dog should be walking to heel by your side on a proper lead, attached securely to a suitable collar or harness. That way, you have control and can ensure the safety of both your dog and other street users. Loose lead walking is a skill and it can be learned, but as other posters have said, you'll never learn it while your dog is on extending lead as it effectively teaches the dog to maintain a slight "pull" at all times and that pulling is rewarded by allowing the dog to go where it likes. For loose lead walking your dog should be at your side, with the lead hanging down under the neck forming a "smile" shape.
Our newest dog arrived a month ago with no lead walking skills and he is now walking nicely on the lead - I walk him and our two others on the lead together singlehandedly. For recall, running around etc. we either use our indoor dog park or we hire a secure field - there is an excellent FB page which lists secure dog fields all over the UK and is very useful when you are working with a new dog.
While you are teaching recall, use a long line. And if you are somewhere dangerous, eg. livestock nearby, I simply prefer to walk my dogs on the lead. That way, we can all have a safe walk without bothering anyone else. We use a 6ft lead routinely, so I can use that for both close heelwork and more relaxed ambling/sniffing.
We have personally experienced numpties who have tripped up other walkers, wrapped their leads round our dogs' legs and caused accidents to cyclists. There are many instances of nasty injuries caused by them when they wrap round legs and even taking off tops of fingers.
If you drop them while your dog is running the noise/weight of the handle part can further frighten the dog, thus making it more likely they'll run.
Whenever we see them being used at our local NT place, popular with "weekend" dog walkers, we give them a wide berth. They are invariably used by people with limited awareness of their surroundings who are quite happy to gas on their mobile while the dog is tripping up innocent pedestrians 30 feet away.
Responsible dog ownership starts with ensuring the safety and wellbeing of your dog, but it must also include an awareness of the impact of dog ownership on other people sharing public spaces. That means having your dog safely under control - something which is much more difficult using one of these contraptions. It takes a bit more effort, but learning loose lead walking and reliable recall are both achievable and will ensure you will never be one of "those" dog owners that so many threads here are started about.