I think Flexi leads have their time and place, but most people who use them are clueless dog owners who think their dog needs 20ft of freedom on a leashed neighbourhood walk. The majority of people who use them don't use them in a controlled, proper way. They let the dog roam all over the place, making their dogs a nuisance to traffic, a nuisance to others walking, etc. I've also heard of quite a few Flexi Leads breaking under pressure, because the dog pulls too much with them.
I personally have no need for a Flexi, and don't use them. I prefer to keep my dog controlled on a fixed length loose leash beside me. When I do need a long lead, I put my dogs on a long line, and only ever in an open field where they can't bother anyone. I think the Flexi can be a good tool in proper hands, but overall I don't like them just because so many ignorant people use them on their unruly dogs.