I occasionally use a nylon cord type Flexi, I did find the thing very useful when walking on fells or steep areas where allowing a varied amount of length on a leash helps when negotiating steep and difficult paths. I used it when we approached sheep and although Brandy was very good in the fields with sheep there was only one occasion when I needed to restrain her and the Flexi did the job.
I have tried handling more than one dog with each on a Flexi.......

big mistake.....although I'm not bad at untangling knots, the springers defeated me regularly and having to untangle your legs whilst chatting to a passer by is embarrassing
Make sure the cord does not stick and retracts easily, some worn out units can be a source of problems.....you really do need the cord to retract freely. Make sure it will lock in a short length easily too then you can use it as a short lead too.
I agree that the cord type can cut and give a nasty friction burn and I suppose the tape type will be better...I have never tried one.
Julie can't get on well with the Flexis so she never uses them on the dogs.
Steve