It is complimentary with ESP, it backs up everything ESP does, it should actually reduce the need for intervention by the ESP system meaning that the electronic nannies will not feel to intervene as much and thus save you some money on brake pads.
Unlike ESP a proper LSD will help you corner faster and will give you better traction.
The implementation of ESP varies from car to car, but I think it is usually more than just traction control.
If all the SSS has is traction control the helical LSD will again reduce the need for this system, as it will reduce the skidding of your wheels under power.
On a side note, there are 5 SSSs rolling around quite happy on my LSDs, to my knowledge no one has hit a tree yet