Hi John,
I have been diving with a Freedom 7 for a while. I have not had a run-in with a big shark while wearing it (of course the day I didn't wear it I had a huge Zambezi have a few goes at me!) It has spooked a few gully sharks, but I have not had any shark experinces to speak of while diving with it.
I find that on shallow dives in caves, it tended to fall down around my head while I am head down in a cave, and in the surge for cracker, it also often ends up whipping past your pip, giving you a nice wake-up while you are trying to be nice & quiet. And in deeper water however I try to flare out and land on the reef, it tends to fall over my other leg and shock me there. I have resolved all of that by drilling 3 holes in the outside of my fin and putting cable ties through them which I leave open enough to fit the shield, and thread it through. Got this tip from an old time cape diver, Geoff Fridjohn, and it works a treat.
I have felt more comfortable with it than without it, although to be honest after that brute zambezi, it doesn't feel like anything short of a cage would really stop a big shark.
I only dive with it upcoast, in Cape Town kelp diving you don't need one, and you very seldom hear of whites while hunting tail, plus it would be a huge extra hassle while trying to get onto and into a school of tail. And no real shark issues in the deep for tuna, all of which is really the only CT diving I do.
Cheers,
Tony