I always say that instead of a knee to the groin, I would get into a position to strongly come up straight between the legs with my hand in a grab position and not only JAB sharply, but grab ahold and TWIST. I know that many attackers get their victims to the ground and this would be perfect for that position. I think that you'd have at least a couple of minutes to get free if that were to happen.
I like to have it on my wrist on an elastic stretchy key chain thing...that way its "out there".
I have set up a few friends with this that travel alone as I do...those late night rest stops and hotels, parking lots...one friend was traveling w another lady from MN to TX for a dog show...they pulled over at a rest area...she went to use the facilities and HEARD a group of guys talking about how hot she was while in the restroom alone. I chewed her out and told her to IGNORE the no dog signs...we both have Bullmastiffs..my life is worth a citation or warning about taking a dog into a restroom. I gave her my wrist held spray for the trip home.
I used mine when the neighbor had 2 nasty dogs that kept digging out and attacking everyone...the stuff works ..just be careful to not leave it on a hot car.
I prefer a pistol these days however...but pepper spray is excellent if the wind isn't misdirecting it.
Wasp spray is a great sub to keep around..and if your really serious, most gun shops sell bear spray.