I've used it before,switching between that and IO and never had any problems.IMO,they can be used interchangeably as they are from the same company and i'd assume they have similar mineral/element levels.This has been my experience when using both salts when one or the other isn't availiable.I do know from testing that ReefCrystals has a higher calcium level than IO but i've never tested either for mag.I have been using both brands now for the past yr and have never experienced stressed corals from using either.
That said,i still dont get the positive reaction from my corals when doing WC with NSW.Something about the organics/plankton in it that make my corals happy.My problem is i'm too lazy to collect it otherwise i'd use it exclusively.