When someone has a concern about their dog’s leash walking, generally one of my first questions is “What kind of equipment are you using?” By equipment I mean leash and collar/harness. Both of those items can make or break your walk with your dog.
Across the board I advise people to use regular nylon or leather leads and not retractable leashes (ie: the Flexi Leash) for walks. They do have their place since they allow for a greater amount of freedom for your dog and in certain safe situations that is appropriate.
One thing to always remember when training your dog is that they will continue to do a behavior that gets rewarded. On a retractable leash, the dog is rewarded for pulling forward because he gets to go where he wants faster. If you put a dog that is used to a Flexi on a nylon lead, he will try to pull your arm clean off!
For leash walking they can be very dangerous. While your dog can get more freedom, it is next to impossible to suddenly give them less freedom. Because they are so thin I have heard of cases where a driver saw the person walking, but did not see that he had a leash and therefor had no idea there was a dog ahead. Those situations end badly for the dog.
I have also heard horror stories of how Flexi Leashes have injured people. A Dogster article pointed me to the illustrations at the left about some of the dangers. All of those illustrations are actually from the Flexi Leash company website.
Many trainers want extendable leashes banned all together but I am not one of them. However, it is a tool that must be used only when it is safe and appropriate.