I know that they are thought of as evil things, and that many people would like to see them banned, but I do think that flexi leads have their uses.
The thing with a flexi lead is that it is like any other tool we use, be that a harness, collar, head collar or long line. On their own, they are likely to cause more problems than they solve. However, with a bit of training they can serve a useful purpose.
I wrote about Vinnie and his numerous issues in a previous post. He's recently been going through a flare up of his symptoms - there were subtle signs that he just wasn't as comfortable as he had been. After discussing it with his acupuncturist we agreed that it was likely caused by his antics whilst off lead on a walk. He has no self preservation - he goes off spanielling with no thought whatsoever as to what that is doing to his body. Adrenaline kicks in and he's invincible. So I needed to find a compromise; restrict him, but allow him some freedom to just be a dog. I don't want him to have to spend the rest of his days on lead only exercise, and I want to keep him off pain medication as long as possible. Of course I will start pain medication when he needs it, but we're not there yet.
Enter the flexi lead. I've used this on and off with Vinnie over the years. As with any lead, he knows exactly how long it is. He's not really a puller, but does like to have the length of a lead (a TTouch or Mekuti harness with a two point lead has really helped to get him walking better for the sake of his whole body!) So that's rule number one with the flexi - he doesn't generally race out to the full length and keep moving. It puts just enough pressure on his harness that he knows he's attached, and thus restricts himself accordingly. Rule number two is right there - it's always attached to a harness and NEVER a collar. Because dogs are unpredictable - if a rabbit or pheasant were to race across his path then he may well shoot out to the end, and I don't want him doing that on a collar.
Vinnie has a pretty decent recall - another rule of the flexi for me. I don't often use it to reel him in - if I need him closer to me, then I just call him back. I can of course apply the brake if needed, but try not to without warning. He also knows that going the wrong way around trees or any other object is a pain. He's the one that has to untangle himself, so he generally avoids getting tangled in the first place. He also has excellent directional cues; I can say 'left' or 'close' and he will move left or right as needed.
I'm under no illusions about the limited control that the flexi lead gives me. If I see another dog coming, and I'm unsure as to how he may react, then I will call him back and if needed get hold of his harness. He can be a bit unpredictable these days - he's had some bad experiences, as well as periods of un-diagnosed pain. I'd rather he wasn't put in a position he's not comfortable with so I manage him accordingly. I also don't use a flexi lead for any sort of controlled walking (e.g. road walking) And I wouldn't use it in busy places; he just has to stay on lead in those cases, maybe allowing him the six foot length to have a mosey.
Here's a little video clip of Vinnie on his flexi lead on a walk. This is pretty typical of how he is throughout the walk (as long as we aren't anywhere near water)
So anyway, that's my thoughts on flexi leads, and why I choose to use one with my dog. As I said at the start, the key is understanding that it's just another tool and that there is some training required in order to get optimum use.