i have one in my basement, not hooked up. it is the salt version.
i have talked to multiple water companies as well as places like the water depot and others that sell purification and softening devices. the answer i have gotten is that it is ok for aquariums, the salt that makes it into the waterlines is so miniscule that it isn't noticeable nor threatening. according to them the salt would be gone from the water used within 45 min. basically it would be less overall salt content by far than what we have dosed our tanks with to fight off infections and other health issues with our rays.
it isn't tasted in the water from the tap from what i'm aware.
i would like to hear from others who do have personal experience with the salted water softeners though, just for the fact that they have done it. did it work? did it cause problems for the fish?