I think it will depend entirely on how hard/soft your water is (we have rather softish water here), and what kind of soil you are dealing with. Soap nuts work best in warm/hot water.
I do pretreat any sort of grease stain (bacon grease, usually), often with dishsoap. It's probably not going to get out grass stains, or wine, without a little more help.
What we have discovered, is much less contact dermatitis, less lint, and normal "body odor" type soil washes out completely. Clothes smell "fresh" without any sort of perfuminess.