Since ich "lives" in the tank with a host, it seems to me to do a FW dip only rids that fish at that moment. When you add back into the tank the parasite still lives and will attack again.
I have tried it twice and both times have been unsuccessful. It is very stressful on the fish. After I did the FW dip the fish just laid on its side breathing very heavily then the next day it died. And like Clarkii said the tank still has the ich parasite so after you put the fish back in the tank it will just get reinfected. IMO hyposalinity is the easiest and safest method for the fish.