Love our flexi leash
We just got one for Magoo last month but I have used one extensivley with an 80 pd german shepard and never had a problem. I did once get burned by a non-retracting 6 foot training leash however.
Our flexi leash is at least an inch wide and is for dogs weighing up to 110 pds, though Magoo is only 55. It cost a little more, but is highly durable. We use it everyday for walking around the neighborhood, we don't have a fenced in yard so Magoo only gets outdoor offleash time a few times a week. I felt horrible that he was glued by my side for his daily walks and not allowed to sniff and explore a little. I keep it locked to the shorter length as we walk and Heel and especially if I see another person or distraction coming, but then let it out as a reward for good behavior when we come to a field or open place he can explore. It works great for us, he knows if he heels for awhile he gets to be free (well sorta free =) ) He will trot out to the end of the leash and look back at me, like "are you coming or what?" He enjoys the extra freedom.
As for injuring myself, it could happen but since it's happened to me with a non-retractable as well, I doubt I would stop using the flexi.