Shock collars are a fantastic tool when used properly. Dogs are very place and situation oriented, and usually don't confused conditioning given for different reasons and different situations, even if the stimulus is the same. For normal retriever training of the type that we do, the shock is very mild, barely a tingle. The only time the a "jolt" is used is when a lesson absolutely must be learned for the dog's own safety...like rattlesnake avoidance and poison-proofing.
And yes, we need to be very careful when training in public. Not only because of PETA types, but because of plain old concerned pet lovers whose attention we don't want to draw, but completely understand.;) We love our dogs like crazy.