Chemspec Stainshield is a highly regarded product that users say performs very well. Chemspec Stainshield professional and Chemspec Enviroshield have a higher percentage active ingredients which makes it more durable. From memory, I think there's about 10% difference between the two which makes the Professional/Enviroshield one of, if not the, most durable in the industry.
I remember on my early years Prochem training that they demonstrated some carpet samples that had been treated with products X,Y and Z and the protectors could be "abraided off" with your hand.
Safe and happy cleaning:)
Ken