We have a 12 lb. Min Pin, and he is very active and fast. The Flexi leash has been great, except for the fact that the opening where the string slides through has been enlarged as the plastic wears, eventually splitting the body of the leash casing, so that the string now gets caught all the time. I would say that started a few months ago, so, we really got about 7 months of good use.
I wish you guys would put a metal aglet, or guide there, so that the plastic doesn't wear.
The matter of using a string, or cord, which could really hurt the dog, took some getting used to, but if one is very conscious of the dog's habits/movement, it's not really a problem.