On the whole, no.
I have used them with small dogs when off-lead time has been restricted (e.g. during the foot and mouth outbreak some years back when all dogs had to be kept on lead) but the majority of people you see out and about don't know how to use them safely.
I've twice been tangled and brought down by other people's dogs on Flexi leads approaching mine, both times out in public and on the street where common sense dictates you shouldn't use one. On one occasion both me and my dog ended up injured; on the other, when the other dog's lead lock was released by the owner so the dog could whizz across a pelican crossing at us, we held up traffic, being honked at, whilst we sorted ourselves out.
If you're going to use one, teach your dog to walk properly on a normal short lead first, then practice somewhere safe with the Flexi so the dog understands it.
A Flexi is likely to teach your dog to pull; there is always tension on a Flexi (there has to be, or it would not retract properly) and the dog's opposition reflex will cause it to pull against that tension.
All those people who get a Flexi for little Alfie or Mollie puppy so they 'can have some freedom' from day one tend to end up with a puppy that makes poor choices, frightens itself or gets frightened and becomes reactive, pulls stranglingly badly or who is chaotic on a lead at best.
As others have said, it isn't the tool necessarily causing the problem but the workman. This particular tool, though, is ubiquitous and looks like it's a far better idea than it actually is if you don't know what you're doing.
My personal choice is a good, strong hi-viz longline. These also need practice (or at least a training session) to use and can also cause burns and other accidents but as a rule are less likely to be used inappropriately.