Parelli is a method of bonding with your horse gaining respect and obedience through the mind of a horse rather than a human.  It is expensive and whether you use other Natural Horsemanship methods e.g. Monty Roberts, Kelly Marks and many others or Pat and Linda Parelli you will gain invaluable knowledge and pleasure from spending time in a menage with your horse.  In my opinion it will not change the temperament of a horse but assists the bonding process and therefore a better horse/owner relationship.  I believe respect and safety are key to any horse ownership.